To assess the effect of pain on PaO2 in newborns, transcutaneous PO2 (tcPO2) was monitored in ten healthy male infants undergoing circumcision. The tcPO2 was seen to decrease to a lower level with pain and to remain low with continuing pain. After the circumcision, the tcPO2 increased to levels equal to or exceeding those of the preoperative baseline period. Close corcordance of tcPO2 above and below the waist demonstrated no right-to-left shunting through the ductus arteriosus.