To describe the relation between scalp blood flow and transcutaneous PO2 and PCO2, these three signals were recorded simultaneously in eight experiments on four healthy lambs in the first weeks after birth. Scalp blood flow was recorded by a laser Doppler flow sensor that was incorporated in the transcutaneous PO2 electrode. Scalp blood flow was varied by applying a circular pressure on the scalp surrounding the sensors. A decrease in laser Doppler blood flow was associated with a decrease in transcutaneous PO2 and an increase in transcutaneous PCO2, according to a hyperbolic curve. The steep parts of the hyperbolic curves were observed at low scalp blood flow, whereas the flat parts were found at high flow values. Even during heat-induced hyperemia only a small part of these hyperbolic curves showed a reasonable independence of the transcutaneous blood gas values of changes in blood flow. In view of the previously observed decrease in intrapartum scalp blood flow, the present data render the transcutaneous measurement of blood gas values doubtful for fetal monitoring during labor.