The locomotor activity of groups of three CD-1 female mice was increased by 1.0 and 2.0 g/kg ethanol, IP, was decreased during the first hour and increased during the second hour by 3.0 and 4.0g/kg, and was decreased by 5.0 g/kg. The dose (2.0 g/kg) that caused the greatest increase in locomotor activity did not impair motor coordination, measured by the height of aerial righting in mice. Tests after oral administration of ethanol showed that the increase in locomotor activity of mice was not due to peritoneal irritation. The same dose (2.0g/kg) did not increase the locomotor activity of C57BL/6J mice. Ethanol (0.1 to 3.0 g/kg) had no effect or decreased the locomotor activity of individual male Sprague-Dawley rats. These findings suggest that biological differences in strains and species of laboratory rodents contribute to the apparent variability of locomotor stimulation caused by ethanol. The presence or absence of an ethanol-induced increase in locomotor activity was not dependent on the sex or number of mice or rats tested. Intertrial-interval crossing by rats acquiring or performing an active avoidance task in a shuttle box was increased by ethanol. This action was dependent on the presentation of electric foot shock. Apomorphine (0.25 and 2.5 mg/kg) and fenmetozole (7.5 and 15.0 mg/kg) failed to inhibit the ethanolinduced increase in intertrial-interval crossing by rats, although these drugs have been shown previously to antagonize the ethanol-induced increase in the activity of mice ethanol treatment. The ethanol-induced increases in the spontaneous locomotor activity of CD-I mice in photocell activity monitors and in intertrial-interval crosses in rats in a shuttle box task thus do not appear to share a common mechanism.