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Although estrogen-induced prolactinomas have been widely studied, little attention has been accorded to local pressure effects of the tumor on the hypothalamus and portal vasculature. To portray the magnitude of this phenomenon, four groups of 12-13-week-old female Fisher 344 rats were studied. Group 1 was an intact control receiving a subcutaneously (SC) placed placebo pellet; group 2 was an ovariectomized control with a SC placed placebo pellet; group 3 was ovariectomized with a 10 mg SC placed diethylstilbestrol (DES) pellet; and group 4 was ovariectomized receiving both 10 mg DES and 10 mg SC placed bromocriptine pellets. Blood samples were obtained at 4 weeks, and the animals were sacrificed at 8 weeks after pellet implantation at which time blood, pituitary and hypothalami were obtained. At 4 weeks serum prolactin levels were similarly and significantly elevated above the control groups in both the DES and DES/bromocriptine groups. By 8 weeks, however, serum prolactin level(s) in the DES-treated animals had tripled from the 4-week value, while levels in the DES/bromocriptine-treated animals were unchanged from the 4-week values. This finding matched the observation that the DES-treated animals had pituitaries 2.5-fold heavier than the DES/bromocriptine animals. The gross and histologic structure of the hypothalami and portal vessels were markedly disrupted in DES-treated rats and much less so in the DES/bromocriptine-treated group. These findings lead us to speculate that the pathogenesis of DES-induced prolactinomas proceeds in two phases: First, there is an early chemical induction phase in which estrogen directly and indirectly stimulates lactotrope proliferation and, second, a mechanical disinhibition phase, where tumor-induced destruction of the hypothalamus and portal vessels unleashes the pituitary from the dopaminergic restraining effects of the hypothalamus.