To examine differences among pregnant diabetic and nondiabetic subjects, serum glucose, and immunoreactivity of C-peptide, free and total insulin were measured at hourly intervals during a 24--h third trimester metabolic ward evaluation. Six normals, three mild, and four juvenile-onset type diabetics were studied. Diets were identical for all subjects. Mild diabetics differed from juvenile diabetics by having significant residual pancreatic B-cell function, as measured by C-peptide immunoreactivity. Short and intermediate acting insulins given once or twice daily to diabetics maintained serum glucose levels within the normal range throughout the 24 h. Despite wide variation in serum total insulin levels, peripheral free insulin concentrations in well-controlled diabetics fell within a relatively narrow range that was higher than in controls. Infants of the diabetic subjects were comparable to the offpsring of the control women.