The long-term consequences of a single social defeat on open field behaviour in rats were studied, with special emphasis on the time course of stress-induced changes. Animals were subjected to social defeat by placing them into the territory of an aggressive male conspecific for 1 h. After the defeat session experimental animals were returned to their home cage and their own room, receiving no further cues from the resident. Other animals serving as controls were placed in a clean and empty cage for 1 h. Five-minute open field tests were performed on days 1, 2, 7, 14, and 28 after defeat, with independent groups of rats. Locomotion of the animals was recorded and analyzed with an automated video system. Social defeat resulted in a strong subsequent reduction in open field activity, which lasted till at least 7 days after the conflict. Differences in total travelled distance were no longer significant 2 weeks after the conflict. The latency for moving to the outer ring of the open field arena after the start of the test was still significantly longer 4 weeks after defeat. The stress-induced reduction in open field locomotion could be reversed by 12-h sleep deprivation during the resting phase, an intervention known to have antidepressant effects in humans. Possible relevance of the present findings with respect to human affective disorders is discussed.