In cats lightly anaesthetized with cnloralose, small (5 to 50 mug./kg.) intravenous doses of 5-hydroxytryptamine temporarily reduced the flow of urine into the bladder in addition to causing reflex falls of blood pressure and heart rate and temporary arrest of breathing. Doses of phenyl diguanide and other aryl diguanides, the reflex effects of which on blood pressure and respiratory movement approximately matched those of 5-hydroxytryptamine, had comparatively little antidiuretic effect. Phenyl diguanide, unlike 5-hydroxytryptamine (tested previously under similar conditions), did not have an antidiuretic effect in hydrated mice in doses of 0.05, 1, and 2.5 mg./kg. subcutaneously. It is concluded that the reflex depressor action of 5-hydroxytryptamine plays little part in reducing urinary output.