We have determined prehepatic insulin secretion rates (ISRs) in seven patients with gestational diabetes mellitus (GDM) and in eight age- and weight-matched nondiabetic pregnant women during late gestation (third trimester) and again postpartum. Plasma glucose concentrations were raised to ∼8.9 mM with iv glucose (hyperglycemic clamping), and ISRs were determined by deconvolution of peripheral C-peptide concentrations using C-peptide kinetic parameters that were obtained in every patient during late gestation and again postpartum. Plasma insulin levels were measured by RIA with an antibody with minimal (<0.2%) cross-reactivity with proinsulin. During late gestation, women with GDM were more insulin resistant than nondiabetic controls and had significantly lower ISRs (689 vs. 849 pmol/min, P < 0.05) and glucose uptake rates (30.6 vs. 49.4 μmol/kg·min, P < 0.05) in response to hyperglycemia. Postpartum, ISRs and insulin resistance decreased in women with GDM and controls (ISR by 43% and 43%, respectively, and insulin resistance by 75% and 118%, respectively), and both groups had similar ISRs (352 vs. 408 pmol/min, nonsignificant). Women with GDM, however, continued to be more insulin resistant than controls. In summary, patients with GDM during late pregnancy not only had severe deficiencies in ISR but, in addition, were more insulin resistant than controls. Postpartum, insulin resistance and ISRs (and plasma insulin levels) improved in both groups, and ISRs (and plasma insulin levels) were no longer significantly different in patients with GDM and controls. Insulin resistance, however, remained higher in women with GDM, and their glucose uptake remained lower. We concluded that the women with GDM had a major β-cell defect that made it impossible for them to compensate for their increased level of insulin resistance, which occurred during late pregnancy.