Melatonin, a molecule synthesized and secreted by the mammalian (including human) pineal gland, has a variety of seemingly unrelated functions in organisms. In photoperiodically-dependent seasonal breeders, the changing melatonin signal imparts seasonal information to the species thereby regulating the annual cycle of reproduction Melatonin also is involved in a number of 24 h rhythms and is believed to be an important component of the circadian system. More recently, melatonin was found to relate to immune function in organisms and to be an effective antioxidant. As an antioxidant melatonin would appear to provide substantial protection against free radicals which are generated under a variety of experimental corrections, including ischemia/reperfusion injury. These latter two functions of melatonin, i.e., as an immune system modulator and as an antioxidant, both may have applicability to cell and organ transplantation.