 <p>You need to set an expectation on the UsarioService to say what will be returned when usarioService.save(...) is called. </p> <p>Before getting to that point, you need to say in the test mockUsarioService.createMock() which will create the actual instance of the mock object, that's what you will pass to the controller usarioService attribute. Copied the code below from the Grails documenation. <a href="http://grails.org/doc/1.1/guide/9.%20Testing.html" rel="nofollow">http://grails.org/doc/1.1/guide/9.%20Testing.html</a></p> <pre><code> String testId = "NH-12347686" def otherControl = mockFor(OtherService) otherControl.demand.newIdentifier(1..1) {-&gt; return testId } // Initialise the service and test the target method. def testService = new MyService() testService.otherService = otherControl.createMock() </code></pre>