Sex differences in the effects of several drugs of abuse have previously been demonstrated. The present study evaluated sex differences in the effects of acute and repeated dosing with Delta(9)-tetrahydrocannabinol on locomotor activity in male and female Swiss Webster (CFW) mice. Results showed that acute Delta(9)-tetrahydrocannibol (1-30 mg/kg) increased locomotor activity in female, but not in male, mice. Following 3 days of daily injections of 10 mg/kg Delta(9)-tetrahydrocannabinol, re-tests of the same doses of Delta(9)-tetrahydrocannabinol tested acutely showed that female mice continued to exhibit greater sensitivity to the stimulatory effects of Delta(9)-tetrahydrocannabinol than did male mice; however, higher doses were required to produce these increases, suggesting a modest degree of tolerance development. These results are consistent with those of previous studies in which cannabinoids were more potent at producing locomotor suppression and antinociception in female than in male rats.