diff -ru old_rhino1_7R2/src/org/mozilla/javascript/NativeObject.java rhino1_7R2/src/org/mozilla/javascript/NativeObject.java
--- old_rhino1_7R2/src/org/mozilla/javascript/NativeObject.java	2009-03-22 14:32:24.000000000 +1100
+++ rhino1_7R2/src/org/mozilla/javascript/NativeObject.java	2009-04-02 19:24:29.000000000 +1100
@@ -71,6 +71,14 @@
     }
 
     @Override
+    protected void fillConstructorProperties(IdFunctionObject ctor)
+    {
+        addIdFunctionProperty(ctor, OBJECT_TAG, ConstructorId_getPrototypeOf,
+                "getPrototypeOf", 1);
+        super.fillConstructorProperties(ctor);
+    }
+
+    @Override
     protected void initPrototypeId(int id)
     {
         String s;
@@ -256,6 +264,18 @@
               }
               return Undefined.instance;
 
+          case ConstructorId_getPrototypeOf:
+              if (args.length < 1) 
+                  return Undefined.instance; 
+
+              Object arg = args[0];
+
+              if ( !(arg instanceof Scriptable) )
+                  throw ScriptRuntime.undefReadError(arg, "__proto__"); // TODO: create proper error message for this
+
+              Scriptable obj = (Scriptable) arg;
+              return obj.getPrototype();
+
           default:
             throw new IllegalArgumentException(String.valueOf(id));
         }
@@ -303,6 +323,8 @@
     }
 
     private static final int
+        ConstructorId_getPrototypeOf = -1,
+
         Id_constructor           = 1,
         Id_toString              = 2,
         Id_toLocaleString        = 3,