Plasma glucose, insulin, and proinsulin concentrations were measured before and after an oral glucose challenge in 57 nondiabetic individuals. In addition, insulin-mediated glucose disposal was estimated by determining the steady state plasma glucose (SSPG) concentration after a 180-min iv infusion of somatostatin, insulin, and glucose. The plasma glucose concentration after oral glucose administration was used to divide the population into those with normal (n = 36) or impaired glucose tolerance (IGT; n = 21), and the 36 normal glucose-tolerant individuals were further subdivided into an insulin-sensitive (SSPG, < 9.0 mmol/L; n = 15) and an insulin-resistant (SSPG, > 10 mmol/L; n = 21) group. Fasting and postglucose load insulin concentrations were similar in the normal glucose-tolerant insulin-resistant and IGT groups, but were significantly higher (P < 0.02- < 0.001) than those in normal glucose-tolerant insulin-sensitive individuals. Fasting proinsulin concentrations were also higher (P < 0.002) in the normal glucose-tolerant insulin-resistant (15.1 +/- 1.5 pmol/L) and IGT (15.8 +/- 1.8 pmol/L) groups compared to those in normal glucose-tolerant insulin-sensitive volunteers (9.3 +/- 1.2 pmol/ L). However, the ratio of fasting proinsulin to insulin was identical in all three groups (0.12). When the three groups were combined, significant relationships (P < 0.001) existed between SSPG (degree of insulin resistance) and both fasting proinsulin (r = 0.59) and insulin (r = 0.66) concentrations, but not with the ratio of proinsulin to insulin (r = 0.03). These results demonstrate that fasting proinsulin and insulin concentrations are increased in insulin-resistant, nondiabetic subjects, and the more insulin resistant, the greater the increase. In contrast, the ratio of proinsulin to insulin did not vary as a function of insulin resistance. Thus, neither insulin resistance nor the need to secrete more insulin to maintain glucose tolerance necessarily leads to abnormal insulin processing by the beta-cell.