The fetal acid-base balance from scalp blood in the early and late first stage of labor and from cord arterial blood was assessed in 120 low-risk patients only monitored electronically for 15 minutes at admission. The mean scalp blood pH values were 7.332 at a cervical dilatation of 5 cm and 7.335 at 10 cm cervical dilatation. A low pH value (less than 7.25) was found in 14 patients; five (4.2%) of them had fetal distress. Stethoscopic auscultation failed to identify these cases. Another nine patients had transient abnormal acid-base balance (a check value within 30 minutes was normal), and the fetal heart rate trace (obtained immediately after the sampling) showed no ominous changes in these cases. It is concluded that an abnormal fetal acid-base balance is not uncommon in a low-risk population in labor. However, a critical evaluation of the acid-base balance is necessary, since in most cases the abnormal acid-base balance is transient and of a respiratory, innocuous type without concomitant fetal distress.