Hypophyses of four groups of 50 female rats (F0--nulliparous, F0--multiparous, F1--standard diet, and F1--low protein) were examined by light microscopy and immunocytochemistry by the avidin-biotin complex method. In pituitary glands of rats fed a low protein diet, the incidence of neoplasms of the pars distalis, the number of multifocal tumors, and the degree of neoplastic cellular atypia were significantly reduced. Pituitary adenomata in rats fed a low protein diet also caused significantly less brain compression at the time of necropsy than in rats fed a standard diet. All but two neoplastic foci from the 83 tumors stained for prolactin, and 14 of these also stained for growth hormone. No differences were found in the frequency of hemorrhagic or solid patterns of tumor. These findings may reflect a reduction in pituitary tumor initiation, promotion, and/or progression in rats fed a low protein diet.