Somatomedins have been shown to have potent mitogenic activities in cultured cells and also promote individual growth. We studied the concentrations of somatomedin C in diabetic pregnant women and rats. Serum and liver somatomedin C were measured by RIA double antibody method, after extraction with an ODS silica column. Serum somatomedin C concentrations were 20.45 +/- 5.14 nM/l (mean +/- S.D.) in nonpregnant normal women. In nonpregnant diabetic women, these were 16.96 +/- 4.37 nM/l, which were significantly lower than those of normal nonpregnant women. The concentrations of pregnant normal and diabetic women were similar to those of nonpregnant normal women. Maternal concentrations of somatomedin C significantly correlated with infant weight (r = 0.41, p less than 0.05, n = 41). Serum somatomedin C concentrations in the normal infants were 6.45 +/- 1.97 nM/l, which were considerably lower than those of their mothers. In the infants from diabetic mothers, these were 9.17 +/- 3.28 nM/l, which were significantly higher than those of normal infants. Rat serum somatomedin C concentrations which were 15.86 +/- 2.37 nM/l in nonpregnant normal rats, tended to increase during pregnancy. The mother's concentration significantly correlated with fetus weight (r = 0.491). Liver somatomedin C levels decreased in diabetic and pregnant states. Fetal rats had lower hepatic somatomedin C, which was about 10% of the mother's levels. These were higher in the fetuses from diabetic mothers. These finding suggested that the mother's somatomedins might contribute to fetal growth.