Eleven healthy, full-term babies were studied on the second day (d2) after birth and again 4 weeks (w4) later. The babies lived in natural lighting conditions and were fed every 4 h. Blood pressure, heart rate, skin (abdomen) and rectal temperatures were measured at 10-min intervals for 24 h. Behavioural states (deep sleep, light sleep, drowsy, alert, crying) were measured at the same times, and used to purify the raw data. Both the raw and purified data were assessed for circadian (24-h) rhythmicity by cosinor analysis. Circadian rhythms in heart rate and blood pressure were poorly developed at d2 and w4. By contrast, skin and rectal temperatures showed circadian rhythmicity that increased in amplitude between d2 and w4; this increase was seen in both raw and purified data. The masking effect due to sleep changed also; the depression caused by "deep sleep" became greater between d2 and w4. The results indicate that the development of circadian rhythmicity in body temperature was not secondary to that of a circadian pattern of sleep and activity, and presumably derives from a body clock that is beginning to develop independently.