The effect of an intravenous infusion of secretin (2.0 CU/kg/h) on serum prolactin (PRL) and estradiol levels and plasma levels of vasoactive intestinal polypeptide and somatostatin (SRIH) was studied in 8 healthy and normally cycling women during the midfollicular phase (cycle day 7), at midcycle (day 14), and during the midluteal phase (day 21) of the menstrual cycle. When compared to basal preinfusion levels, a significant decrease in serum PRL levels was observed at steady state concentrations of plasma secretin (+ 30 to + 60 min) both during the follicular (p < 0.03) and the luteal (p < 0.0001) phases. At midcycle a nonsignificant decrease was observed. A significant and negative correlation existed between serum PRL and plasma secretin levels in the follicular phase (r = ––0.33; p < 0.05) and in the luteal phase (r = ––0.73; p < 0.0001). The plasma concentrations of SRIH increased significantly at steady state conditions of secretin at midcycle (p < 0.02) and in the luteal phase (p < 0.04), while no effect was found during the follicular phase. A significant and positive correlation between plasma levels of SRIH and secretin was observed at midcycle (r = 0.63; p < 0.002) and in the luteal phase (r = 0.46; p < 0.02). No effect of secretin on plasma vasoactive intestinal polypeptide and serum estradiol concentrations was demonstrated. These results suggest that the suppression of PRL in the follicular phase of the spontaneous menstrual cycle can be ascribed to an effect of secretin alone. In the luteal phase, however, there is evidence for an additional effect of SRIH, and indirectly possibly also progesterone, to cause a more pronounced inhibition of PRL release.