The effects of 6-chloromelatonin and 6-fluoromelatonin on ovulation and LH [luteinizing hormone] and prolactin [PRL] 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 LH but did not alter the proestrous PRL surge, nor did they alter basal serum levels of LH or PRL. The plasma half-life of 6-chloromelatonin in rats and rhesus monkeys was 27 min compared to 11 min for melatonin. 6-chloromelatonin is a melatonin agonist with greater metabolic stability than melatonin.