The body temperature, heart rate and rest-activity cycles of 12 low-risk preterm infants (29-35 weeks) were continuously recorded in the neonatal intensive care unit over a period of 1-2 weeks. Throughout the recordings the light was on continuously, the feeding was done intra-gastrically every 2 h and the incubator temperature was constant. Under such conditions a significant circadian rhythm was found in the body temperature and heart rate of more than 50% of the infants. These findings indicate the possible existence of an endogenous circadian rhythm in early human infancy. Since lights are continuously on in the neonatal intensive care unit, this might impair the time of emergence of day-night entrainment of the biological clock in preterm infants.