The time of maturation of the circadian periodicity in humans has been differently considered. The present study aimed to investigate the existence of rhythmic variations in the body temperature of healthy full-term infants just after birth. We studied 19 healthy term newborns, nursed in their cribs at environment temperature of 25 degrees C and moderately dimmed artificial lighting during the night. Continuous recording of body temperature was performed with a solid memory recorder (Fiamarker) connected to a disposable rectal probe, during the first three days of life. Data were analyzed by means of single and mean cosinor methods and spectral analysis. All the newborns, except two, demonstrated a statistically significant circadian periodicity of temperature (p < .001). Acrophases were distributed along the 24h since the synchronization to environment was not yet completed. A clear ultradian fluctuation of body temperature was observed in all 19 newborns with an unexpected fall of temperature every three-four hours. Our data show that the maturation of the circadian system is probably almost complete in newborns, but the adjustment to the new environment can be expected in the subsequent weeks of life.