These experiments studied changes produced by a hypnotic dose of ethanol in the LS and SS lines of mice, which differ in ethanol sensitivity. In the first experiment, animals were injected either with ethanol or saline, and activity and seizure susceptibility measured 7-9 h later when blood levels of ethanol would have reached zero. Ethanol-treated mice of both genetic lines were less active in an open field test and more susceptible to clonic convulsions induced by flurothyl than saline-injected controls. There was no difference in the magnitude of these changes in the two lines. In the control condition SS (short-sleep) mice were more active than LS (long-sleep) mice, and more susceptible than LS mice to myoclonic but not to clonic seizures. The effect of the ethanol injection on body temperature was evaluated in separate groups of animals. LS mice showed a more pronounced hypothermia than SS mice when temperature was measured 2 h after injection. Six hours after injection, SS mice exhibited a small but statistically significant overshoot in temperature, after which they again became hypothermic with respect to controls; hyperthermia was not observed in LS animals.