The administration of alcohol to male mice 2–6 h before the preparation of second meiotic metaphases from testes resulted in an approximately six-fold increase in aneuploidy. The timing employed indicates that the observed chromosome abnormalities were a result of nondisjunction and/or anaphase lagging at the first meiotic division. A similar effect has been described in the female mouse; however, the present results suggest that the aneuploidy-inducing effect of ethanol may be substantially greater in the female than in the mal