In many species of vertebrates the pineal gland and its indoleamine hormone melatonin play central roles in the control of circadian rhythms, whereas in some species, the pineal gland appears to hold little importance. However, recent research indicates that the circadian rhythms of many species of reptiles, birds and mammals, including humans, are synchronized by the administration of exogenous melatonin. These studies have led to questions concerning the role of this hormone in circadian organization in general. Studies of the sites and mechanisms of melatonin action further indicate that melatonin may be an excellent pharmacological tool for research on the cellular mechanisms of circadian clock function and have pointed to the possibility that melatonin or melatonin analogues may be therapeutically useful for the control of circadian clock dysfunctions such as jet lag, shift-work syndrome and sleep disorders.