Catecholamine histofluorescence was used to examine morphology of hypothalamic tuberoinfundibular dopaminergic (TIDA) median eminence-afferent neurons (areas A12 and A14) in male Fischer 344 rats bearing prolactin (PRL)-secreting tumors. These tumors were either spontaneous, discovered in rats aged 20-32 months, or were induced by chronic diethylstilbestrol (DES) treatment in younger animals. In the latter case, histofluorescence was examined in two groups of animals: (1) those which had been treated, beginning at 6 months of age, with 8 weeks of DES, and subsequently had had the treatment discontinued for 10 months, but which continued to exhibit hyperprolactinemia; (2) animals treated continuously with DES, for 50 or 70 days. Histofluorescence was induced using either the Falck-Hillarp, aluminum-formaldehyde ('ALFA') or the formaldehyde-glutaraldehyde ('FAGLU') technique and pituitaries were examined for PRL immunoreactivity, as permitted by the fixation method chosen for histofluorescence. For selected animals, pituitaries were enzymatically dissociated, maintained in primary culture, and were assessed for PRL cell quantification, intracellular PRL content, and in vitro release. Among animals treated chronically with DES, histofluorescence was assessed by the Falck-Hillarp technique, and counts of A12 and A14 perikarya were made in each hypothalamus. Median eminence zona externa fluorescence was diminished in aged rats with normal pituitaries, in aged rats bearing spontaneous tumors, and in rats treated continuously with DES, in comparison with fluorescence in younger rats with normal pituitaries or from which DES treatment had been with-drawn. Perikaryal fluorescence showed typical cell morphology and numbers in aged rats, aged rats bearing spontaneous tumors, and in rats from which DES had been withdrawn. In one animal, treated continuously for 70 days with DES, numbers of A12 and A14 perikarya decreased. In aged rats bearing spontaneous tumors, histofluorescence morphology indicated stimulatory effects on TIDA perikarya and terminals, including increased perikaryal and terminal fluorescence intensity. The morphological results support biochemical evidence of PRL stimulation of tuberoinfundibular dopaminergic neurons, and suggest that inhibitory effects of increased PRL secretion on these hypothalamic cells in DES-induced hyperprolactinemia reflect a direct and reversible effect of DES on these neurons.