The plethysmo-time-interval (PTI) is the time interval between the beginning of the QRS complex on an ECG and the upstroke of the pulse wave on a plethysmogram, as measured by pulse oximetry. In the present study, we investigated homeothermal acclimatization to the extra-uterine environment in human neonates using the Coretemp thermometer and a pulse oximeter. Temperature was measured at three sites: the central deep temperature (CDT) on the upper sternum, the peripheral deep temperature (PDT) on the flat part of the left sole of the foot and the surface temperature (ST) at the side of the abdomen. After delivery, CDT and ST were higher than PDT. The difference between CDT and PDT was large at first, but gradually decreased. PDT, initially in the range of 32.4 +/- 0.28 degrees C, reached a stable value (34.4 +/- 0.41 degrees C) at 2.5 h after delivery. PTI was prolonged in parallel with PDT. The difference between CDT and PDT probably reflected the contraction of skin vessels, particularly the arterioles, which occurs as a body defense mechanism against heat loss. As PTI was prolonged in parallel with PDT, we demonstrated objectively that this catch-up phenomenon of PDT after delivery was affected by the increase in skin blood flow as a result of dilatation of peripheral arterioles. It was concluded that PTI can be used to evaluate the peripheral circulatory status of the neonate, even during homeothermal adjustment after birth, by applying a new principle of pulse oximetry that is widely used in neonatal intensive care units.