Alcohol consumption (both acute and chronic) and alcohol withdrawal have a variety of chronobiological effects in humans and other animals. These effects are widespread, altering the circadian rhythms of numerous physiological, endocrine, and behavioral functions. Thus, some of alcohol's negative health consequences may be related to a disruption of normal physiological timing. Most studies of alcohol's chronobiological effects have been conducted under natural conditions in which environmental stimuli, such as regular cycles of light and darkness, act to coordinate circadian rhythms with the environment and with each other. However, such studies cannot distinguish between effects occurring directly on the circadian pacemaker and those occurring "downstream" from the pacemaker on the physiological control systems. Studies using animals have enabled researchers to begin to examine the effects of alcohol on circadian rhythms under so-called free-running conditions in experimental isolation from potential environmental synchronizers. These studies have provided preliminary evidence that alcohol's chronobiological effects are indeed the result of direct influences on the circadian pacemaker itself. Furthermore, the effects of alcohol on animal circadian rhythms appear similar to the effects seen during administration of antidepressant drugs. Taken together with evidence that the chronobiological effects of alcohol withdrawal in human alcoholics are reminiscent of those described in depressed patients, these observations suggest that alcohol may produce antidepressantlike effects on the circadian pacemaker. One theory suggests that the effects of alcohol on the circadian pacemaker are mediated in part by alterations in serotonin, an important chemical involved in cellular communication within the circadian system. However, other neurochemical systems also are likely to be involved.