Immunocytochemistry (ICC) and reverse hemolytic plaque assays (RHPAs) for growth hormone (GH) and prolactin (PRL) were used to investigate the ontogeny and regulation of GH- and PRL-secreting cells in the rat. GH-containing cells were first found in day 19 fetuses, yet only half of these initial somatotropes were able to release GH under basal or stimulated conditions. PRL-releasing cells were first detected one week later in neonatal rats, 4 days after birth. By employing a sequential plaque assay that enabled the detection of GH and PRL release from the same cells, it was demonstrated that almost all of the initial PRL secretors on day 5 also released GH. Treatment of pituitary cultures from 5-day-olds with selected hypothalamic factors for six days produced chronic effects on the proportions of cells that secreted GH and PRL. The RHPA for GH was then used to investigate the mechanism(s) underlying the sexual differences in GH release which evolve at puberty in rats. After showing that the absolute number of GH-releasing cells in male and female rats was equal at 10, 20, 30, 50 and 100 days of age, it was possible to demonstrate that sexual differences in GH release are attributable to the secretory capacity of individual somatotropes. In addition, by studying hormone release from individual cells it was shown that insulin-like growth factor-I (IGF-I) acutely prevents the release of GH by a sub-population of somatotropes in the adult rat, and that somatostatin attenuates the release of GH by most, if not all, somatotropes. This evidence suggests that somatostatin is a much more effective inhibitor of total GH release than IGF-I. Finally, a RHPA for casein release from isolated mammary cells was employed to demonstrate that liver tissue of lactating rats produces a potent lactogen which partially mimics the actions of PRL. This material increases the amount of casein released per mammary cell, yet has no effect on the number of cells committed to casein release.