In order to better understand the role of A- and B-cell function in diabetic pregnancy, we studied four groups of pregnant women at week 34-36 of gestation. Seventeen were healthy controls (C), 24 had gestational diabetes (GD), 16 had type 2 diabetes (NIDD) and 37 had type 1 diabetes (IDD). At times -20, 0, 20, 30, 45, 60, 90 and 120 min from the beginning of a 30 min infusion of 30 g of arginine intravenously, plasma glucose, glucagon (IRG) and C-peptide (CPR) were measured. Plasma glucose was higher in diabetic than in control subjects. IRG values were also higher in the GD and the NIDD women. CPR values were similar to, or slightly higher than control values in the GD and the NIDD and were much lower in the IDD women. All three variables increased during the arginine infusion in all groups, with the exception that CPR remained unchanged in the IDD. The CPR/IRG molar ratio was similar in control, GD and NIDD women; in the IDD, it was much smaller than in the other groups and was not affected by arginine. In all the diabetic patients, IRG was negatively correlated with the maternal weight gain and in the IDD IRG was positively correlated with the increase in the insulin need and with the CPR levels. In conclusion diabetes appeared to enhance the A-cell function also in pregnancy, possibly impairing the 'facilitated anabolism' and stressing the 'accelerated starvation' which are typical of normal pregnancy. Glucagon was confirmed as one possible determinant of the insulin resistance seen in diabetic pregnancy.