An action of dopamine on kidney function has been reported by McDonald, Goldberg, McNay & Tuttle (1964) in man and by McNay, McDonald & Goldberg (1963) in dogs. We have studied the effect of dopamine (D) (3,4 dihydroxyphenylethylamine HCl; Nutritional Biochemical Co.) on diuresis in rats. Male rats (200–300 g.) of the Instituto strain were used. They were trained for 3 days to minimize the effect of stress and observations made both during water diuresis and at normal rates of urine flow. Water diuresis was induced by giving tap water by stomach tube (4 ml./100 g. body wt). After placing the rats in a special container, urine was collected every 30 min. for 2 hr. and the bladder was gently pressed before and after each 30-min. period to ensure complete evacuation. Dopamine (2 mg./100 g. body wt) was administered i.p. in 0·2 ml. saline. In five rats with normal rates