Polycystic ovary (PCO) syndrome, characterized by excess androgen and reduced estrogen is a major cause of infertility. It is the most common endocrine disorder with a prevalence of 8-17% in women of reproductive age worldwide. Ricinus communis (RC) has been reported to have estrogenic property as well as anti-androgenic property as such it may have some effects on the endocrine disorder associated with PCO. The study was aimed at investigating the effects of Ricinus communis seed oil on letrozole-induced PCO in Wistar rats. Forty female animals were grouped into four (n=10). The negative Control (NC), Positive Control (PC), Treatment Groups (TG) 1 and 2. NC received 2mL/kg body weight (bwt) 1% CMC orally and was not induced with PCO, while PC, TG1 and TG2 were induced with PCO through 21 days of oral administration of 1mg/kgbwt letrozole dissolved in1% CMC (Kafali, 2004). The method causes an increase in testosterone and a decrease in estrogen. After the induction phase, NC and PC received distilled water while TG1 and TG2 were treated with 500mg/kgbwt and 1000mg/kgbwt RC seed oil (orally) respectively for 21 days. Animals were thereafter sacrificed at proestrus through cervival dislocation. Estrous cyclicity was studied in each animal throughout the study. Data (mean±SEM) analyzed (by student’s t-test, P<0.05) include: estrous cycle length and percentage estrous phase; serum levels of testosterone, estrogen and FSH; Weights and histology of ovaries and uteri. Uterine weight (g) was decreased (P<0.05) in PC, TG1 & TG2 (0.56±0.03, 0.64±0.13, 0.41±0.0) when compared with negative control (1.18±0.11). Estrogen (pg/ml) was reduced in the PC (20.25±1.20) and TG2 (10.90±3.04) as compared with NC (66.85±10.87) (P<0.05). Testosterone (nmol/l) was increased (p<0.05) in PC (3.00±0.24) and TG1 (3.27±0.41) as compared with NC (1.98±0.14). A significant increase was seen in the %estrus phase of the PC (27.61±2.33) and TG2 (38.08±5.40) during treatment as compared to during induction period (PC=15.23±1.85; TG2=18.56±3.13). Cycle length increased in PC (8.67±0.92), TG1 (10.50±0.64) and TG2 (7.86±0.58) during induction as compared with pre-induction period (4.13±.10, 4.09±0.07, 3.93±.06) but decreased during treatment period (6.73±0.11, 7.17±0.41, 5.57±0.24). Histology of the PC showed cystic spaces and degenerating follicles. TG1 showed few cystic spaces. NC and TG2 showed normal ovarian stroma. The result showed that estrous cycle was prolonged during induction of PCO and characterized by less estrus phases which was improved by the treatment of 1000mg/kgbwt RC. Testosterone in this group could also be compared to the NC however the estrogen levels were still significantly lower. The results showed that 1000mg/kgbwt RC seed oil may reduce the excess androgen and histopathological changes caused by PCO in Wistar rats.