Ovine prolactin injected subcutaneously for 2 days stimulated an increase in fluid uptake in vitro by everted gut sacs from male Sprague\p=m-\Dawley rats. Four intestinal segments from each rat were incubated, but only two of the four segments tested showed a prolactin-stimulated increase in fluid transfer in intact rats. In adrenalectomized\p=m-\nephrectomized rats all four sacs showed an increased fluid transfer after prolactin treatment. Sacs incubated for 1 h in glucose\p=m-\Ringer solution containing 0\m=.\1 mg ovine prolactin/ml did not show an increased water transfer as compared with controls incubated in medium containing albumin. Dietary changes seemed to alter the sensitivity of the gut to prolactin.