The physical fitness of 120 healthy primigravidae was determined two weeks before term using first the standardized three-stage submaximal work test on the following day the voluntarily maximal pulse-conducted work test on a bicycle ergometer. After the first test the level of lactic acid in the capillary circulation of 115 mothers was examined. Immediately after delivery the pH and the level of lactic acid in the maternal artery, umbilical vein and umbilical artery were determined. The level of lactic acid after the work test was negatively correlated with the physical performance of the mother. The level of lactic acid in the mother after the work test and the levels of lactic acid in the umbilical vessels were positively correlated. The mean pH value in physically fit women after delivery was almost significantly higher than in the women of below average performance. The pH level in the umbilical artery was also almost significantly higher in the mothers with above average performance. The physically fit women appear to work more during delivery than the less fit mothers, as the level of lactic acid after the delivery was as high as or higher than in mothers with a physical performance below average. The physical fitness of six mothers, who delivered an asphyxiated baby, was almost significantly lower than the performance of the mothers. Five mothers with an exceptionally high pH after delivery had a significantly higher performance than the other mothers.