Variability of fetal scalp blood flow was measured in 15 patients in labour. Using the continuous transcutaneous laser Doppler technique adapted for fetal intrapartum measurements, it was possible to obtain recordings from a cervical dilatation of 3 cm until delivery. The general trend of fetal scalp blood flow showed a decrease in the majority of the patients during the first and the second stages of labour, with a fall of more than 50% of the initial value at the end of the first stage in seven. During the second stage the decrease in flow was generally less pronounced, although in one fetus scalp blood flow fell to 18% of the initial level by the end of the second stage. Of all the uterine contractions recorded, 57% were accompanied by a transient decrease in blood flow, often by 40-50%, but a temporary increase in scalp blood flow was found in 25% of contractions. Caput succedaneum formation was associated with a more pronounced transient decrease in local scalp blood flow during contractions. It is concluded that during labour a considerable reduction in fetal scalp blood flow occurs. It follows that the reliability of fetal transcutaneous blood gas monitoring techniques, which need a substantial increase in local capillary blood flow, seems highly questionable.