In an acute fetal lamb model the relation between arterial PCO2 (PaCO2), transcutaneous PCO2 (tcPCO2) and pH was studied at different conditions of stress. Occlusion of the maternal common iliac artery for 8 min, umbilical cord obstruction for 5 min and placental embolization were performed subsequently, every time with an interval of 1 h for fetal recuperation. During the first 2 experiments arterial values changed rapidly after occlusion and returned nearly to normal within 30-60 min after the end of occlusion. TcPCO2 started to increase several minutes after occlusion and reached its maximum about 5 min after the end of occlusion. Afterwards a gradual decrease towards the onset value was observed. During placental embolization tcPCO2 did not increase until 15 min before fetal death. It is concluded that tcPCO2 can follow a gradual trend, but not quick changes in PaCO2.