Abstract: The impact of oral administration of 100 mg kg 1 b.w of chloroform extract of Carica papaya seed (CPE) on oestrous cycle, fertility and serum 17 -oestradiol levels, in female rats was investigated. Ten proestrous rats received 2.5% tween 80 in normal saline (vehicle for CPE) and served as the control in each part of the study. In the oestrous cycle study, ten proestrous rats were treated with CPE for 14 days. The phases and frequencies of the oestrous cycles of the rats were determined daily for another 14 days while CPE treatment continued. In the fertility study, ten proestrous rats were treated as in the oestrous cycle study for 14 days and were thereafter cohabited with fertile untreated male rats for another 14 days while CPE treatment lasted. CPE did not adversely affect body weight of the rats. However there was a significant decrease (p<0.05) in the weight of the ovary, but not in the uterus. There was a significant decrease (p<0.01) in serum 17 -oestradiol levels in CPE treated rats. The oestrous cycle became irregular, with prolonged diestrous phase from the 3rd week of CPE treatment. There were disorganization and degeneration in the ovary. The uterus showed signs of vacuolation and mild disorganization. The extract treated rats produced a significant decrease in litter number (p<0.01) but the fetal weight and morphology remain unchanged relative to the control. The results suggest that chloroform extract of Carica papaya seed has antifertility properties, possibly acting via inhibition of oestrogen secretion.