Laser Doppler flowmetry was used to evaluate the relationship between the changes in scalp blood flow and transcutaneous PO2 (PtcO2) during labour. Laser Doppler flow (LDF) and PtcO2 were measured simultaneously in fourteen women using a combined sensor. Scatter diagrams of LDF and PtcO2 show a hyperbolic curve, i.e. at high flow levels PtcO2 is independent of the flow rate and will reflect the arterial PO2 value, whereas at lower flow levels PtcO2 becomes highly dependent on the blood flow rate. The latter situation occurred in most of the fetuses when labour progressed. An inverse relationship between LDF and PtcO2 was observed in two fetuses in those parts of the recordings which coincided with repetitive severe variable decelerations++ in the FHR tracing (r = -0.59 and -0.65), suggesting a compensatory increase in scalp blood flow during hypoxemia. It is concluded that the PtcO2 signal is considerably influenced by local scalp blood flow in the majority of the patients and thus is a questionable parameter for monitoring fetal carotid arterial PO2.