Forty patients with moderate symptoms of PD will be recruited and randomised to receive Exenatide injections twice daily, or to act as controls in this open label trial. Detailed assessments will be made of all patients at baseline and periodically for a total of 14 months. The primary outcome measure will be the change between baseline and follow up, in the severity of a validated PD assessment scale (the UPDRS part 3 motor score) after an overnight period free of conventional PD medication. Secondary measures will include adverse event reports, self completed questionnaires, and blood test results. Aside from these assessments, all patients will continue their regular PD medications throughout the trial with adjustments made only according to clinical need.