Maternal and foetal acid-base balance, PaO2, lactate, potassium and creatine phosphokinase (CPK) were studied during the course of 28 induced labours. Every second mother received segmental epidural analgesia during the first stage of labour (epidural group), while the remaining mothers (who were given pethidine for pain relief, if necessary) acted as a control group. In the epidural group the patients had only minimal changes in acid-base balance and lactate concentration during the first stage. During the second stage lactate concentration increased. In the control group, on the other hand, the acid-base balance showed signs of hyperventilation and lactic acid accumulation during the first stage. The potassium changes were quite minimal and were not significantly different between the groups. The CPK level did not change during labour, but 2 and 4 h after delivery it was significantly elevated in both groups. The foetal acid-base balance, potassium, lactate and PaO2 values revealed no differences between the groups at any time. The CPK level in umbilical venous blood was significantly higher in the epidural group.