Abstract Objective: We investigated lactate concentrations in fetal scalp and cord blood to determine the sources of fetal lactacidosis in fetuses with ominous heart rate patterns. Methods: Cord blood was collected from newly delivered infants who had been monitored by fetal scalp blood sampling during labor. In 250 cases umbilical arterial and venous cord blood lactate levels were measured. We assessed the umbilical arterial lactate concentrations in relation to the venous lactate levels, the arterial pH level, base excess, and arteriovenous lactate differences in cord blood. In 103 cases the levels of lactate in fetal scalp blood, sampled within 60 minutes of delivery, were compared with those in the umbilical artery and vein and the pH level and base excess immediately after birth. Results: Lactate level in the umbilical artery showed a significant correlation to that in umbilical venous blood ( r = .84, P < .001), to arteriovenous lactate differences ( r = .52, P < .001), as well as to pH ( r = −.55, P < .001) and base excess ( r = −.63, P < .001) in arterial cord blood. Lactate concentrations in fetal scalp blood shortly before delivery showed a significant correlation to lactate levels in the umbilical arterial ( r = .65, P < .001) and venous blood ( r = .62, P < .001). Conclusion: The study indicates a close correlation between lactate levels in arterial and venous cord blood, as well as between the lactate levels and pH and base excess in cord arterial blood in patients with ominous fetal heart rate patterns. We also found an increased fetal contribution with increasing lactacidemia. Lactate concentrations in fetal scalp blood correlated well with those in cord arterial and venous blood.