AbstractWe conducted experiments to determine the effects of chloroquine (CQ) on the oestrus cycle and ovulation in 4‐day cyclic rats. There were two treatment groups. Group I animals had chloroquine phosphate (40 mg kg−1body wt.) administered intraperitoneally (i.p.) starting from dioestrus day 1, once a day, 5 days a week for 4 weeks. Oestrus smear was monitored by daily saline vaginal lavage. Control rats received an equal volume of physiological saline. All animals were sacrificed at the end of the 4th week of treatment.Group II animals also received chloroquine phosphate (40 mg kg−1body wt.) administered in a single dose at either 0900 h or 1800 h pro‐oestrus, and on the morning of oestrus the rats were killed. Trunk blood was collected in each group at the time of sacrifice, centrifuged and the serum stored for subsequent radioimmunoassay of luteinizing hormone (LH), follicle stimulating hormone (FSH) and oestrogen. The fallopian tube was dissected out and a search made for the ova.Results showed that administration of chloroquine altered the oestrus cycle (i.e. the rats showed a persistent dioestrus smear), lowered serum oestrogen and luteinizing hormone levels while serum FSH was unaltered, prevented the expected ovulation when injected at 0900 h pro‐oestrus and did not affect ovulation in rats injected at 1800 h pro‐oestrus. This study supports the notion that chloroquine has an adverse effect on hypothalamo‐pituitary ov