Hourly release of Prolactin by pituitary constructs 1 whole pituitary (PI), adenohypophysis (P-N) and pituitary-hypothalamus co-incubate (PHC) were compared. Adenohypophysis secreted significantly more prolactin than PI and PHC, while PHC secreted significantly less than PI. Co-incubation of (P-N) with posterior pituitary reduced the elevated secretion of prolactin. Addition of dopamine (10(-7) M), bromocriptine (10(-7) M) and apomorphine (5 x 10(-8) M) to these constructs did not affect the release of prolactin from PI but inhibited the same from (P-N) and PHC. Treatment with reserpine increased serum prolactin levels but intrapituitary prolactin contents were decreased. Hourly release of prolactin from pituitary constructs derived from reserpine-treated rats was significantly reduced as compared to ascorbic acid--treated controls. Inclusion of dopamine (10(-7) M), bromocriptine (10(-7) M) and apomorphine (5 x 10(-8) M) in these constructs inhibited prolactin secretion further. In vitro addition of perphenazine stimulated the release of prolactin by PHC but was without any effect on PI and (P-N). The data are interpreted to suggest that dopamine in posterior pituitary may be an important determinant of hypothalamic modulation of prolactin secretion.