AbstractGenetic analysis is revealing molecular components of circadian rhythms. The gene products of the period gene in Drosophila and the frequency gene in Neurospora oscillate with a circadian rhythm. A recent paper(1) has shown that the PERIOD protein can undergo both intermolecular and intramolecular interactions in vitro. The effects of temperature and two period mutations on these molecular interactions were compared to the effects of the mutations and temperature on the in vivo period length of circadian rhythms. The results suggest that the molecular interactions may compete to maintain a rhythm with a constant period over a wide temperature range.