Fetal scalp temperature and fetal-maternal temperature gradient were studied during well-established labor in a group of 97 patients using a technique that ensured at the same time the adhesion and the thermal insulation of the probe. In 78 infants with a one-minute Apgar score of 7 or above, a positive gradient of 0.2C was maintained throughout labor between the warmer fetal scalp temperature (37.3C) and the maternal rectal temperature. Periodic drops of temperature related to the uterine contractions occurred in only 43% of the cases. In a group of ten infants with a one-minute Apgar score of 6 or below, the fetal-maternal gradient was significantly different from this schema. There was an inversion of the gradient, the fetal scalp temperature becoming cooler (36.9C) than the maternal rectal temperature. The gradient was -0.2C at the beginning of the study and -0.5C 20 minutes before delivery. Periodic drops of temperature with uterine contractions were constant. In dead fetuses, fetal scalp temperature was much below that of the mother and this difference increased steadily until delivery. A good relationship was found between fetal-maternal gradient and the pH of the umbilical artery blood sampled at birth. When the mean gradient was 0.2C +/- 1 SD, pH averaged 7.27; in the group below 1 SD, mean pH was 7.19 (P less than .01).