Ethanol increased the exploratory locomotion of BALB/c mice over a wide dose range (1.15--3.1 g/kg orally), whereas only a 2.31 g/kg dose of ethanol increased the locomotion of C57BL/6 mice. After 1.15 g/kg of ethanol the blood concentrations in BALB/c and C57BL/6 mice were 65 +/- 11 and 68 +/- 13 mg %, respectively, and after 2.3 g/kg--the corresponding concentrations were 156 +/- 26 and 142 +/- 14 mg % (mean +/- SEM). Doses of 3.8 and 4.6 g/kg inhibited the exploratory locomotion of mice of both strains to an equal extent. The induction time and the duration of ethanol-induced anaesthesia, as well as the blood ethanol concentrations (428 +/- 40 and 446 +/- 40 mg %, respectively, at the onset of anaesthesia) were similar in mice of both strains after a 5.4 g/kg dose. However, motor excitement before anaesthesia was observed only in the BALB/c mice. It is suggested that the observed strain differences in response to ethanol are due to low responsivity of C57BL/6 mice to the excitatory action of ethanol and are not caused by differences in the rate of its metabolism. Apparently, the excitatory and anaesthetic effects of ethanol are under separate genetic control mechanisms.