1. The results are given of an experiment comparing the specific locus mutation-rates in male and in female mice having 0.1% caffeine dissolved in their drinking-water up to the age of ten weeks. Their parents had the same treatment from the time of mating, so that the tested germ-cells might be exposed to caffeine during embryonic development. 2. The mutation-rates in males and females did not differ significantly from each other, nor from the known spontaneous rate; thus there was no evidence for induction of mutations by the caffeine treatment. Neither was there evidence for the induction of dominant lethals following caffeine treatment of males. 3. The treatment did not noticeably affect reproduction, but some mice developed aggressive tendencies towards their cage-mates. Some mice were kept on 0.1% caffeine throughout life: they continued to breed satisfactorily on the whole.