Summary. Daily subcutaneous injections of 100 μg melatonin given to prepubertal female rats housed in 14L:10D or 12L:12D failed to delay puberty as evidenced by the age at which vaginal opening occurred; neither the Sprague–Dawley nor the Wistar strain rats were responsive to melatonin treatment. Reproductive organ weights (ovaries and uteri) at vaginal opening were unaffected by such treatment. Administration of melatonin through the drinking water in doses of 100, 500 or 1000 μg/day did not alter the timing of puberty or the reproductive organ weights in rats of the Sprague–Dawley or Long–Evans strains (housed in 12L:12D). Our experimental methods are identical to a previous report and we have no explanation for our failure to reproduce the earlier results.
Keywords: melatonin; puberty; vaginal opening; rat