This study assessed the effects of raloxifene, a selective estrogen receptor modulator (SERM), on ovarian morphology and circulating hormone levels in rats. Female Fischer-344 rats (65/group) were given dietary raloxifene for 6 months at average daily doses of 0, 15, 75, and 365 mg/kg. Morphologic evaluation of ovaries was conducted on 25 rats/group at the end of the treatment period and from 20 rats per group after 1 and 3 months withdrawal from treatment. Plasma hormone analyses were conducted on 10 rats pergroup at the end of the treatment period and aftereach withdrawal period. Treatment with raloxifene for 6 months resulted in disruption of the hypothalamic-pituitary-ovarian axis, manifested by increased plasma concentrations of luteinizing hormone (LH) and estradiol-17beta (E2), and failure of ovulation, manifested by ovarian follicular prominence (retained anovulatory follicles), lack of corpora lutea (CL), and depressed plasma progesterone (P4). Many (56% to 80%) rats in all raloxifene treated groups had focal, minimal to slight hyperplasia of granulosa cells within individual retained follicles. A few treated rats in the mid- and high-dose groups (2 of 25 and 3 of 25, respectively) had more extensive focal proliferation of granulosa cells. These foci were approximately 3 to 6 mm in overall size and were characterized by moderate papillary proliferation of large granulosa cells associated with cystic spaces, often with hemorrhage. In 4 of the 5 rats with this focal cystic granulosa cell hyperplasia, the remainder of the involved ovary and the contralateral ovary were atrophic. After 1 or 3 months of drug withdrawal, most previously treated rats examined had morphologic evidence of ovarian cyclic changes. including developing follicles, various stages of CL, and normal plasma levels of LH, E2, and P4. Continued lack of cyclic changes was limited to 4 of 20 rats from the low-dose group after 1 month of recovery and to 1 low dose rat after 3 months. Intrafollicular granulosa cell hyperplasia was not seen in rats in the reversibility phase. Areas of prior focal cystic granulosa cell hyperplasia were represented by focal sclerosis that included hemorrhage and/or hemosiderin. The foci of sclerosis were associated with cystic spaces after 1 month and were solid after 3 months. A granulosa cell tumor, approximately 12-13 mm diameter, was present in a high-dose rat in the 3-month reversibility group. This tumor effaced 1 ovary and was characterized by proliferative granulosa cells, usually in papillary formations and cords within cystic spaces. This rat had atrophy of the uninvolved ovary, excessive plasma levels of E2 and prolactin, and high P4 levels considering the absence of CL. The results of this study indicate that ovarian granulosa cells in rats are susceptible to proliferative changes when stimulated chronically with excessive trophic hormones. Most of these proliferative changes were reversible upon cessation of the hormonal stimulation. However, the proliferative lesion in one treated rat progressed to apparent autonomous (neoplastic) growth.