The effect of graded doses of intravenous secretin (0.5, 1.0, and 2.0 CU · kg−1 · h−1) on serum prolactin and estradiol levels, as well as plasma vasoactive intestinal polypeptide and somatostatin levels was studied in 6 normally cycling and healthy women, and compared with the hormone levels obtained by a control infusion with physiologic saline (0.15 mol/l). A significant decrease in serum prolactin concentrations was found with increasing doses of secretin at steady-state levels of plasma secretin (+30 to +60 min). A significant negative correlation (p<0.007, r=−0.2764) existed between serum prolactin and plasma secretin concentrations at steady-state conditions. No effect of graded doses of secretin was observed on serum estradiol levels and plasma concentrations of vasoactive intestinal polypeptide and somatostatin. The results suggest a dose-related inhibitory effect of secretin on prolactin release in women.