Following i.v. injection of 50 g of Partusisten into the mother there occurs, already within 5 min., a rise in the lactate concentration which reaches its peak after 25--30 min. After 25 min, a similar can be shown in the fetus. The fetal pO2 at this time has risen significantly to 2.1 mmHg, while the pCO2 has fallen by 3 mmHg. The increase in lactate concentration both in mother and fetus alters the acid base balance in both. pH is reduced and base deficit is raised, parallel in mother and fetus. The fetal acidosis is probably due to a lactate infusion from the mother.