Between 5 and 20% of the adult population in Western countries suffer from insufficient and/or unsatisfying sleep, often associated with certain psychiatric disorders or with certain types of professional activities (for example, shift workers) and travel schedules (for example, jet lag). The benzodiazepines are at present the drug treatment of choice for the management of anxiety and stress-related conditions as well as insomnia. Benzodiazepines are thought to act by potentiating the action of the neurotransmitter gamma-aminobutyric acid (GABA), a widely distributed transmitter in the central nervous system. The circadian system has a key role in the regulation of the sleep-wake cycle, and at least some forms of insomnia may be the result of a disorder of the circadian sleep-wake rhythm. Similarly, at least some forms of depression may also involve disruption of normal circadian rhythmicity. A central pacemaker for the generation of many circadian rhythms in mammals, including the sleep-wake cycle, appears to be located in the suprachiasmatic nucleus, and recent research indicates that both cell bodies and axons containing GABA are present within the bilaterally paired suprachiasmatic nuclei. These findings raise the possibility that the benzodiazepines, commonly prescribed for sleep and mental disorders, may have an effect on the central circadian pacemaker. Here we report that the acute administration of triazolam, a short-acting benzodiazepine commonly prescribed for the treatment of insomnia, induces a phase-shift in the circadian rhythm of locomotor activity in golden hamsters. This suggests a role for GABA-containing neurones in the mammalian circadian system.