Motor incoordination in the rota-rod test was used to assess the development of rapid tolerance to Delta(9)-tetrahydrocannabinol and rapid cross-tolerance between ethanol and Delta(9)-tetrahydrocannabinol in mice. Further, the influence of the cannabinoid receptor antagonist SR 141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide) on the motor impairment induced by both drugs was examined. Mice were injected on day 1 with equipotent doses of Delta(9)-tetrahydrocannabinol (28 mg/kg, i.p.) and ethanol (2.25 g/kg, i.p.) and tested at 30, 60 and 90 min after the injections. On day 2, control groups received ethanol or Delta(9)-tetrahydrocannabinol, some groups received the same treatment as the day before, while the remaining groups switched the treatment. All groups were tested to evaluate tolerance. The development of rapid tolerance to Delta(9)-tetrahydrocannabinol was observed and pretreatment with ethanol resulted in rapid cross-tolerance to Delta(9)-tetrahydrocannabinol. SR 141716A (2 mg/kg, i.p.) failed to block the development of rapid tolerance to both drugs, ethanol and Delta(9)-tetrahydrocannabinol. These results suggest that Delta(9)-tetrahydrocannabinol, similarly to ethanol, can induce rapid tolerance to motor incoordination in mice. They also support the use of the 2-day protocol as an effective procedure to reduce the length of drug exposure necessary to induce tolerance.