This study was designed to examine the transplacental effects of diethylstilbestrol (DES) in rhesus monkeys (Macaca mulatta). Nineteen pregnant females received an approximate human therapeutic dose of 1 mg/day DES beginning on gestational day 21 (Group I), day 100 (Group II), or day 130 (Group III) until term. Colposcopic examination and vaginal biopsies were performed in eight female offspring at 6- and 12-month intervals beginning at 3.5 years of age until death between 5 and 12 years of age. Gross and histological evidence of vaginal adenosis was observed in five of eight (62.5%) females. Adenosis, which occurred in all treatment groups, did not develop into neoplasia; rather, it underwent metaplasia and reversion to squamous epithelium, a frequent observation in human DES cases. The vaginal ridging and/or cervical hooding characteristic of the human syndrome was observed in all exposed females. Although menstrual cyclicity was not impaired, there appeared to be a lower pregnancy rate in treated animals compared to age-matched controls. This long-term evaluation of reproductive morphology and function in the rhesus monkey has provided a useful model for studying the history of benign vaginocervical abnormalities induced by prenatal DES treatment.