1. The effects of acute administration of 3,4-methylenedioxymethamphetamine (MDMA; "ecstasy") on anxiety tested in the light/dark box were examined in albino male mice of the OF.1 strain. 2. Animals were evaluated in the light/dark test 30 min after injection of MDMA (1, 8, and 15 mg/kg, i.p) or saline. The following parameters were recorded (for 5 min); (a) number of exploratory rearings in the light and dark sections; (b) number of transitions between the lit and dark areas; (c) time spent in the light and dark areas; (d) latency of the initial movement from the light to the dark area, and (e) locomotor activity in light area. 3. MDMA (8 and 15 mg/kg) produced a significant reduction in exploratory activity (rearings and transitions), without decreasing motility, in comparison with saline-treated mice. However, time spent in lit/dark compartments was not significantly affected by the drug, which could be a consequence of the anti-exploratory properties of MDMA. 4. Overall, the behavioral profile found in the light/dark test indicates an anxiogenic-like activity of MDMA in mice. It is suggested, however, that animal models of anxiety which emphasize a social interaction could be more sensitive to the effects of this substance.