The effects of 6-chloromelatonin and 6-fluoromelatonin on ovulation and LH and prolactin release in rats were determined. Both halogenated melatonin analogs were more potent ovulation blockers than melatonin. The halogenated melatonin analogs inhibited the ovulatory surge of luteinizing hormone (LH) but did not alter the proestrous prolactin surge, nor did they alter basal serum levels of LH or prolactin. The plasma half-life of 6-chloromelatonin in rats and rhesus monkeys was 27 min compared to 11 min for melatonin. The results indicate that 6-chloromelatonin is a melatonin agonist with greater metabolic stability than melatonin.