The effect of insulin on the transport of proline has been studied in cultured fibroblasts from normal individuals, non-insulin-dependent diabetic patients, and patients with Werner syndrome. In fibroblasts from normal individuals and those with diabetes mellitus, incubation with 10(-7) M insulin resulted in more than a twofold increase in the transport rate after about 14 h incubation. In contrast, fibroblasts from patients with Werner syndrome had a markedly attenuated response to insulin, suggesting a defect in insulin action on the transport of this amino acid in Werner syndrome.