All living and most fossil representatives of the reptilian subclass Archosauria lack pineal bodies. Arrhythmic, low-level, nonpineal melatonin is present, however, in the blood of Alligator mississippiensis . Although pineal bodies have been implicated in circadian phenomena, these results suggest that arrhythmic melatonin in alligators may not be involved in circadian events and indicate that the pineal is not the only source of the hormone melatonin. The evolutionary loss of the pineal in Archosauria occurred during the Mesozoic, an era noted for its seasonal stability. Arrhythmic melatonin titers in alligators and pineal loss in alligators and other archosaurs may be related to Mesozoic seasonal stability.