Creatine kinasebrain isoenzyme activity (CK-BB) was measured longitudinally in the serum of 31 pregnant women in the first stage of labor (early and advanced), at delivery, and 1, 6 and 24 h after delivery, in the umbilical cord and in the serum of their neonates on the first day of life. There was no increase in serum CK-BB values of mothers that delivered normally (n = 15) or had an elective cesarean section (n = 5). Pregnant women with signs of fetal distress had an increase in CK-BB levels in the first stage of labor (mean ± SD 4.5 ± 4.9 U/l, p < 0.03) and 6 h after delivery (12 ± 4 U/l, p < 0.0001). Neonates with intrauterine stress also had an increase in their CK-BB to 144 ± 116 U/l at 6 h of life, in comparison with babies born without signs of stress. It appears that CK-BB during labor and in the first hours of life may be indicative of intrauterine str