One of the best documented effects of melatonin in mammals is its antigonadotrophic effect. Melatonin has been shown to inhibit LH release and ovulation in rats and we chose this model to investigate the effects of new melatonin analogs. The analogs studied include 6-chloromelatonin, 6-fluoromelatonin, and 2-methyl-6,7-dichloromelatonin. All derivatives were more potent than melatonin and appeared to act on the brain and not on the pituitary to inhibit LH release. An exceptionally long duration of activity was found for 2-methyl-6,7-dichloromelatonin. The greatly increased stability of this derivative may provide a valuable tool in search for the role(s) of melatonin in mammals.