An electrode for continuous transcutaneous carbon dioxide (tcPco2) monitoring was attached to the fetal scalp in 25 consecutive vaginal deliveries. In six patients the electrode fell off or was deliberately removed when the fetal head was on the pelvic floor, while eight patients were delivered with the electrode in situ, making it possible to study changes in the fetal carbon dioxide level when the fetus changed from placental to lung breathing. The mean recording time was 136 minutes (range 10-348). Fifteen patients were recorded until delivery and analysis of cord blood showed a statistically highly significant correlation between tcPco2 in both umbilical venous and arterial blood. The mean fetal tcPco2 during the late first stage of labor was 56 torr (range 40-75; n = 18). The post partum tcPco2 level in newborns was stable 20 minutes after delivery with a mean level of 51 torr (range 40-60). Fetuses with a normal fetal heart rate tracing showed a lower mean tcPco2 level than fetuses with fetal heart rate tracing abnormalities. The experience with the current technique indicates that both Apgar score and fetal heart rate tracing are rather crude methods to judge fetal blood gas status.