Recent studies have demonstrated that oxytocin (OT) stimulates prolactin (PRL) release from the anterior pituitary gland and that the secretion of OT into pituitary portal blood changes during the rat estrous cycle. To better define the role of OT on PRL release during the reproductive cycle, the effect of administration of antiserum specific for OT on preovulatory PRL secretion in female rats was studied. Intravenous injection of OT antiserum into cyclic female rats between 13.30 and 14.00 h of proestrus neutralized the elevated levels of OT in pituitary portal blood and significantly reduced the subsequent PRL surge. The characteristic proestrous surge of luteinizing hormone (LH) was not affected by the OT antiserum treatment. These data support a physiological role for OT in the regulation of PRL release during the reproductive cycle.