The effect of melatonin on the growth of B16 mice melanoma was examined. Male and female BALB/c athymic mice, inoculated with 7 X 10(4) melanoma cells, were given drinking water containing melatonin (5 micrograms/g body weight/day) and 0.5% ethanol. Compared to control animals the melatonin treated male and female athymic mice had significantly smaller tumors on Day 40. The weights of the testes, the ovaries, and the adrenal glands of melatonin treated mice were significantly reduced compared to control animals. These data indicate that melatonin p.o. significantly inhibited the growth of B16 mouse melanoma and that the antitumor effect of melatonin was associated with a significant decrease in gonadal and adrenal weights.