A test that was common in the 1960's and was used to detect primary open angle glaucoma was the water drinking test (patients drank 1 quart of water to stress the fluid drainage mechanism of the eyes. Intraocular pressure was then measured over the next hour). Recently this test has been demonstrated as a predictor of peak eye pressure during the day. The purpose of this study is to compare the results of the water drinking test between eyes that have undergone trabeculectomy and eyes that have undergone tube shunt surgery.