Neurohypophyseal oxytocin (OT), secreted continuously under conditions of hyperosmolality, is a potent natriuretic hormone in rats. In contrast, OT secretion during lactation is pulsatile and is not accompanied by increased urinary Na+ excretion. The present experiments compared the effects of continuous and pulsatile infusion of OT on natriuresis in rats. In male rats anesthetized with Inactin, continuous infusion of OT (125 ng/kg x h) increased plasma OT to about 70 pg/ml; renal Na+ excretion increased 10-fold, and urine volume and K+ excretion also were elevated. However, when OT was administered i.v. in the same amount but in pulses given once every 5 or 10 min, to simulate the pattern of OT secretion during lactation, rats did not excrete significantly more urine, Na+, or K+ than did vehicle-treated animals. The plasma renin concentration, measured in these experiments because OT receptors are present in the macula densa, increased 2-fold when OT was infused either continuously or in pulses. These results indicate that the effects of OT administration on urinary Na+ excretion in rats varies depending on whether the infusion is pulsatile or continuous, whereas the effects of OT on renin secretion show no such difference.