Plasma amino acid concentrations were measured in maternal peripheral vein and in umbilical vein and artery at birth (caesarean section) in 6 diabetic and 5 non-diabetic pregnancies. The mean birth weight in the control group amounted to 3.9 kg as oversize of the foetus in three cases contributed to the indication for caesarean section. The infants in the diabetic group consisted of "small for gestational age" babies (mean weight 2.8 kg). Free amino acid levels in the normal group and in diabetic maternal blood were in agreement with previous investigations. No difference in amino acid concentrations in the maternal plasma was found, but the concentrations of the umbilical vein plasma were significantly lower in the diabetic group. Foetal hyperinsulinaemia may be a cause of the low amino acid concentrations. Besides, abnormalities of the placenta and maternal vascular complications increase and the mean birth weights decrease significantly through the White classes. Conditions of impairment of placental transfer of amino aicds may thus be present. Characteristics features of the foetus may be consistent with the hypothesis as follows: The foetus in diabetic pregnancy is in varying degree exposed to an oversupply of glucose, hyperinsulinaemia, imbalanced uptake and a slightly diminished supply of amino acids.