A method has been developed for long-term continuous recordings of several physiological variables in humans, including body movements, core temperature, heart rate and respiratory rate. This method enabled us to study circadian rhythms of body temperature and rest-activity in preterm infants. In 5 out of 9 infants (recorded at 28-34 weeks conceptional age) circadian rhythms were found in body temperature. We conclude that circadian rhythmicity is present during the early neonatal period in certain physiological variables (body temperature) but not in others (rest-activity).