The light-dark cycle is the major synchronizing agent for circadian rhythms in animals. After an abrupt shift in the light-dark cycle, it usually takes many days for circadian rhythms to resynchronize. A single injection of the short-acting benzodiazepine, triazolam, to hamsters subjected to an 8-h advance of the light-dark cycle resulted in an approximately 50% reduction in the time taken for the circadian locomotor activity rhythm to be resynchronized to the new lighting schedule. These results suggest that it may be possible to use drugs to facilitate the resynchronization of human circadian rhythms following an abrupt change in environmental time.