RESUMEN
The return of ovulation after melatonin treatment and the possible effects of its administration on some reproductive indices in mice are reported in this study. Admnistration of melatonin in a dose of 4 mg [1 mg/injection] at 1,2,3 and 4 pm on the proestrus phase of the oestrus cycle resulted in - complete inhibition of ovulation during the cycle of melatonin treatment as judged by absence of vaginal plug in the successive 4 days after injection and absence of corpora lutea on the fifth day and by the histological examination of the ovaries which showed the presence of medium sized follicles and absence of corpora lutea. These results showed also that in most animals treated with melatonin inhibited ovulation for one cycle and the animals ovulated in the next cycle after melatonin treatment. The copulatory rate in the first cycle after melatonin treatment for one cycle was 75% and reached 100% in the second cycle. It was also shown from the results of this study that melatonin led to a non significant increase in the duration of the oestrus cycles, no change in the pregnancy rate and overall pregnancy rate and a non significant change in the duration of full term pregnancy and the number of offsprings in comparison with the control animals. It can be concluded that melatonin inhibited ovulation for one cycle. Melatonin proved also to have no adverse effect on many reproductive indices in mice