The mutagenicity of melatonin and its major metabolite 6‐hydroxymelatonin were evaluated using the Ames test and three strainsof Salmonella typhimurium‐TA 97, TA 98, and TA 100. Neither compound exhibited mutagenicity whether in the presence or absence of an activation system derived from rats induced with Aroclor 1254. Positive controls were employed throughout and gave the expected response. We conclude that melatonin, 6‐hydroxymelatonin, and their microsomal metabolites are not mutagenic in the Ames