Bromocriptine (CB 154), a potent agonist upon the D-2 receptor, blocks the proliferation of the mammotrophs and the elevation of serum prolactin concentration induced by chronic exposure of male Fisher 344 rats to diethylstilbestrol (DES). Fluphenazine, a dopamine receptor antagonist, blocks these effects of CB 154. We conclude that a D-2 receptor occurs on the proliferating mammotroph of the male Fisher 344 rat and that stimulation of this receptor inhibits prolactin release and stops the DES-induced proliferation of these cells.