Morning administration of melatonin antagonizes the antigonadotrophic effect of afternoon injections of melatonin, 5-methoxytryptophol and arginine vasotocin in intact mice.

In intact prepubertal mice kept under a long photoperiod subcutaneous afternoon (4:30 p.m.) injections (20 micrograms) but not morning (9:30 a.m.) injections (100 micrograms) of melatonin and 5-methoxytryptophol elicited antigonadotrophic effects. A morning dose (100 micrograms) of methoxytryptophol abolished the antigonadotrophic effects of afternoon injections of methoxytryptophol. Similarly the antigonadotrophic effects of afternoon injections of melatonin, 5-methoxytryptophol and arginine vasotocin could be partially destroyed by 100 micrograms and completely abolished by 1 mg melatonin administered in the morning.