Increasing evidence indicates that melatonin, the principal hormone secreted by the pineal gland, influences the function of diverse neuroendocrine and other systems in mammals. This indoleamine hormone is also involved in maintaining brain homeostasis, entraining biological rhythms and coordinating reproductive function to changes in photoperiod, particularly in seasonal breeders (Brown, and Niles, 1982; Tamarkin et al, 1985; Reiter, 1991). Other studies indicate a potentially important immunomodulatory role for melatonin (Maestroni, 1993) that binds with high affinity to T-lymphocytes (Gonzalez-Haba et al., 1995). Recent reports that melatonin is a potent free radical scavenger suggest that it may also play an important role as a protective antioxidant hormone (Hardeland et al., 1993; Reiter, 1996).