The plasma concentration of free fatty acids (FFA) was studied in 18 diabetic and in 12 non-diabetic mothers and in their infants. FFA was determined in the maternal and umbilical blood at birth and again in the umbilical venous blood about 3 hours later. At that time an iv glucose load was given, and the FFA concentration was determined after 1 and 2 hours respectively. Furthermore, the FFA concentration was determined on the 5th day of life after 4 hours' fasting and 1 hour after an iv glucose load. The plasma glucose was determined every 10 minutes and K values were calculated at birth as well as on the 5th day of life. The maternal FFA concentration was higher in diabetics, but of the same low level in the 2 groups of neonates. The maternal-umbilical FFA ratio in diabetics was 2.5 as against 1.8 in normal subjects. A positive correlation between maternal FFA concentration and the infants' birth weight was demonstrated in both groups of infants. In infants of diabetic mothers the increase in plasma FFA over the first 3 hours as well as the decrease following the glucose load was much smaller than in the non-diabetic group. On the fifth day of life there was no significant difference between the two groups of infants either in the level of the fasting FFA concentration or in the steep decrease after the glucose injection.