The early development of circadian rhythms in primates, including man, was reviewed. Continuous 24-h recordings were carried out for maternal and fetal circadian rhythms during gestation as well as in preterm infants. Several propositions were made based on these new findings: 1. The fetal biological clock is an endogenous clock capable of generating circadian rhythms and responding to maternal entraining signals long before the moment of birth. 2. Through the fetal biological clock, maternal circadian rhythms influence the fetal overt rhythms. 3. Maternal rhythms influence the fetus, and fetal rhythms feed back to the mother (via the placenta). Disruption of this fetal-maternal interaction during gestation leads to: a. disturbances of maternal and fetal circadian rhythms; b. disappearance of circadian rhythms at the time of birth; c. a gestational period which is either too short or too long (see also Honnebier and Swaab, 1973); d. delayed or impaired maturation of the circadian rhythms of the infant.