We investigated the beta-pancreatic function in response to a standard oral glucose tolerance test (OGTT) and a tolbutamide test in a group of GH-deficient adult subjects and in a group of control subjects. Fasting plasma glucose levels were normal in all subjects; the insulin levels, basal (0.19 +/- 0.02 vs 0.99 +/- 0.08) and, as n-AUC, after OGTT (195.2 +/- 23 vs 520.5 +/- 69) and tolbutamide test (33.6 +/- 4.4 vs 177.7 +/- 12.1) (means +/- ES), were significantly lower (p less than 0.001) in the GH-deficient subjects compared to controls. These data indicate a reduced beta-cell activity, secondary to absence of trophic effect of GH on pancreatic beta-cells, in GH-deficient adults.