diff --git a/lib/defaults.js b/lib/defaults.js
index fb96bdbd13..72c5a0eda2 100644
--- a/lib/defaults.js
+++ b/lib/defaults.js
@@ -58,6 +58,14 @@ var defaults = {
     }
     if (utils.isObject(data) || (headers && headers['Content-Type'] === 'application/json')) {
       setContentTypeIfUnset(headers, 'application/json');
+      if (utils.isString(data)) {
+        try {
+          JSON.parse(data);
+          return data;
+        } catch (e) {
+          return JSON.stringify(data);
+        }
+      }
       return JSON.stringify(data);
     }
     return data;
diff --git a/test/specs/defaults.spec.js b/test/specs/defaults.spec.js
index d71cd66a4e..ab5dcff1aa 100644
--- a/test/specs/defaults.spec.js
+++ b/test/specs/defaults.spec.js
@@ -20,6 +20,18 @@ describe('defaults', function () {
     expect(defaults.transformRequest[0]({foo: 'bar'})).toEqual('{"foo":"bar"}');
   });
 
+  it("should also transform request json when 'Content-Type' is 'application/json'", function () {
+    expect(defaults.transformRequest[0](42, {
+      'Content-Type': 'application/json'
+    })).toEqual('42');
+    expect(defaults.transformRequest[0]('bar', {
+      'Content-Type': 'application/json'
+    })).toEqual('"bar"');
+    expect(defaults.transformRequest[0](JSON.stringify({foo: 'bar'}), {
+      'Content-Type': 'application/json'
+    })).toEqual('{"foo":"bar"}');
+  });
+
   it('should do nothing to request string', function () {
     expect(defaults.transformRequest[0]('foo=bar')).toEqual('foo=bar');
   });