Effects of bromocriptine (CB) on human prolactin (PRL)-secreting adenomas (PRLomas) and growth hormone-secreting adenomas (GHomas) were analyzed morphologically and morphometrically. Treatment with CB for 2 weeks brought about a variety of changes in PRLomas, including cell shrinkage, degenerative and necrotic changes and fibrosis. Secretory granules within a cell increased in number but not in volume. However, the exocytosis of the granules increased remarkably. Single-cell necrosis was occasionally seen in the tumor nests, whereas breakdown of tumor cells occurred in clusters predominantly in the periphery of the nests. At 1 week after cessation of CB, PRLomas showed two distinct, divergent histological appearances; more advanced destructive changes and the regrowth. These findings suggest that human PRLomas consist of two populations, i.e., those sensitive and resistant to the cytocidal actions of CB. In contrast, treatment with CB produced almost no change in GHomas except for an increase in the stromal tissue volume. Vacuolation and single-cell necrosis were occasionally observed in a few GHomas.