diff --git a/dtool/src/dtoolutil/executionEnvironment.I b/dtool/src/dtoolutil/executionEnvironment.I
index 1b4c08e075..52ea458b67 100644
--- a/dtool/src/dtoolutil/executionEnvironment.I
+++ b/dtool/src/dtoolutil/executionEnvironment.I
@@ -51,6 +51,33 @@ set_environment_variable(const string &var, const string &value) {
   get_ptr()->ns_set_environment_variable(var, value);
 }
 
+////////////////////////////////////////////////////////////////////
+//     Function: ExecutionEnvironment::shadow_environment_variable
+//       Access: Public, Static
+//  Description: Changes the apparent definition of the indicated
+//               environment variable by masking it within this class
+//               with a new value.  This does not change the actual
+//               environment variable, but future calls to
+//               get_environment_variable() will return this new
+//               value.
+////////////////////////////////////////////////////////////////////
+INLINE void ExecutionEnvironment::
+shadow_environment_variable(const string &var, const string &value) {
+  get_ptr()->ns_shadow_environment_variable(var, value);
+}
+
+////////////////////////////////////////////////////////////////////
+//     Function: ExecutionEnvironment::clear_shadow
+//       Access: Public, Static
+//  Description: Removes a value set by a previous call to
+//               shadow_environment_variable(), and lets the actual
+//               value of the variable show again.
+////////////////////////////////////////////////////////////////////
+INLINE void ExecutionEnvironment::
+clear_shadow(const string &var) {
+  get_ptr()->ns_clear_shadow(var);
+}
+
 ////////////////////////////////////////////////////////////////////
 //     Function: ExecutionEnvironment::get_num_args
 //       Access: Public, Static
diff --git a/dtool/src/dtoolutil/executionEnvironment.cxx b/dtool/src/dtoolutil/executionEnvironment.cxx
index 878c0e6ffc..7ac74a4882 100644
--- a/dtool/src/dtoolutil/executionEnvironment.cxx
+++ b/dtool/src/dtoolutil/executionEnvironment.cxx
@@ -182,12 +182,12 @@ ns_has_environment_variable(const string &var) const {
 ////////////////////////////////////////////////////////////////////
 string ExecutionEnvironment::
 ns_get_environment_variable(const string &var) const {
-#ifdef PREREAD_ENVIRONMENT
   EnvironmentVariables::const_iterator evi;
   evi = _variables.find(var);
   if (evi != _variables.end()) {
     return (*evi).second;
   }
+#ifdef PREREAD_ENVIRONMENT
   return string();
 #else
   const char *def = getenv(var.c_str());
@@ -215,6 +215,40 @@ ns_set_environment_variable(const string &var, const string &value) {
   putenv(put);
 }
 
+////////////////////////////////////////////////////////////////////
+//     Function: ExecutionEnvironment::ns_shadow_environment_variable
+//       Access: Private
+//  Description: 
+////////////////////////////////////////////////////////////////////
+void ExecutionEnvironment::
+ns_shadow_environment_variable(const string &var, const string &value) {
+  _variables[var] = value;
+  string putstr = var + "=" + value;
+}
+
+////////////////////////////////////////////////////////////////////
+//     Function: ExecutionEnvironment::ns_clear_shadow
+//       Access: Private
+//  Description: 
+////////////////////////////////////////////////////////////////////
+void ExecutionEnvironment::
+ns_clear_shadow(const string &var) {
+  EnvironmentVariables::iterator vi = _variables.find(var);
+  if (vi == _variables.end()) {
+    return;
+  }
+
+#ifdef PREREAD_ENVIRONMENT
+  // Now we have to replace the value in the table.
+  const char *def = getenv(var.c_str());
+  if (def != (char *)NULL) {
+    (*vi).second = def;
+  } else {
+    _variables.erase(vi);
+  }
+#endif  // PREREAD_ENVIRONMENT
+}
+
 ////////////////////////////////////////////////////////////////////
 //     Function: ExecutionEnvironment::ns_get_num_args
 //       Access: Private
diff --git a/dtool/src/dtoolutil/executionEnvironment.h b/dtool/src/dtoolutil/executionEnvironment.h
index dc88bba0ed..33cc896bb3 100644
--- a/dtool/src/dtoolutil/executionEnvironment.h
+++ b/dtool/src/dtoolutil/executionEnvironment.h
@@ -43,6 +43,9 @@ public:
   INLINE static string get_environment_variable(const string &var);
   INLINE static void set_environment_variable(const string &var, const string &value);
 
+  INLINE static void shadow_environment_variable(const string &var, const string &value);
+  INLINE static void clear_shadow(const string &var);
+
   static string expand_string(const string &str);
 
   INLINE static int get_num_args();
@@ -56,6 +59,8 @@ private:
   bool ns_has_environment_variable(const string &var) const;
   string ns_get_environment_variable(const string &var) const;
   void ns_set_environment_variable(const string &var, const string &value);
+  void ns_shadow_environment_variable(const string &var, const string &value);
+  void ns_clear_shadow(const string &var);
 
   int ns_get_num_args() const;
   string ns_get_arg(int n) const;
diff --git a/dtool/src/prc/configVariableFilename.cxx b/dtool/src/prc/configVariableFilename.cxx
index 4fd429de6a..7ae5ac61f2 100644
--- a/dtool/src/prc/configVariableFilename.cxx
+++ b/dtool/src/prc/configVariableFilename.cxx
@@ -17,6 +17,7 @@
 ////////////////////////////////////////////////////////////////////
 
 #include "configVariableFilename.h"
+#include "executionEnvironment.h"
 
 ////////////////////////////////////////////////////////////////////
 //     Function: ConfigVariableFilename::reload_value
@@ -26,7 +27,17 @@
 ////////////////////////////////////////////////////////////////////
 void ConfigVariableFilename::
 reload_value() {
-  _value = Filename::expand_from(get_string_value());
+  nassertv(_core != (ConfigVariableCore *)NULL);
+
+  const ConfigDeclaration *decl = _core->get_declaration(0);
+  const ConfigPage *page = decl->get_page();
+
+  Filename page_filename(page->get_name());
+  Filename page_dirname = page_filename.get_dirname();
+  ExecutionEnvironment::shadow_environment_variable("THIS_PRC_DIR", page_dirname.to_os_specific());
+
+  _value = Filename::expand_from(decl->get_string_value());
+  ExecutionEnvironment::clear_shadow("THIS_PRC_DIR");
 
   _value_seq = _core->get_value_seq();
   _value_stale = false;
diff --git a/dtool/src/prc/configVariableSearchPath.cxx b/dtool/src/prc/configVariableSearchPath.cxx
index 3c4a4a9ef4..a842520cfd 100644
--- a/dtool/src/prc/configVariableSearchPath.cxx
+++ b/dtool/src/prc/configVariableSearchPath.cxx
@@ -47,8 +47,14 @@ reload_search_path() {
   _value.append_path(_prefix);
   int num_unique_references = _core->get_num_unique_references();
   for (int i = 0; i < num_unique_references; i++) {
-    string dirname = _core->get_unique_reference(i)->get_string_value();
-    string expanded = ExecutionEnvironment::expand_string(dirname);
+    const ConfigDeclaration *decl = _core->get_unique_reference(i);
+    const ConfigPage *page = decl->get_page();
+
+    Filename page_filename(page->get_name());
+    Filename page_dirname = page_filename.get_dirname();
+    ExecutionEnvironment::shadow_environment_variable("THIS_PRC_DIR", page_dirname.to_os_specific());
+    string expanded = ExecutionEnvironment::expand_string(decl->get_string_value());
+    ExecutionEnvironment::clear_shadow("THIS_PRC_DIR");
     if (!expanded.empty()) {
       _value.append_directory(Filename::from_os_specific(expanded));
     }