In order to determine insulin status and beta cell function during the oral glucose tolerance test (OGTT), in impaired glucose tolerance (IGT), 51 such subjects and matched controls, identified during a population survey for diabetes, underwent a 75 g OGTT. Fasting, 30 min and 2 h insulin and intact proinsulin, and fasting and 2 h 32/33 split proinsulin, were measured by specific two-site immunoradiometric assays. The subjects with IGT had higher fasting (geometric mean +/- SD, 5.0 +/- 4.0 pmol-1 vs 2.9 +/- 1.7, p < 0.02) and 2 h intact proinsulin (23 +/- 14 vs 14 +/- 12, p < 0.0001), and fasting (3.2 +/- 3 pmol-1 vs 1.8 +/- 1.8, p < 0.0007) and 2 h 32/33 split proinsulin (18.3 +/- 19 pmol-1 vs 6.6 +/- 15, p < 0.0001). Despite higher plasma glucose concentrations, the IGT group had similar fasting insulin, lower 30 min insulin (216 +/- 124 pmol-1 vs 278 +/- 130, p < 0.02), and a lower 30 min insulin/glucose ratio (23.7 +/- 2.1 vs 34.8 +/- 2.3, p < 0.002). The percentage of fasting proinsulin-like to total insulin-like molecules was higher in those with IGT (15.3 +/- 8% vs 11.6 +/- 8, p < 0.04). After 6 months, at repeat OGTT, the same subjects with IGT were classified as 'persisters' or 'reverters'. The persister (24/51 47.1%), at initial OGTT, had a higher 2 h glucose level, a greater BMI and higher systolic blood pressure, but other parameters were similar to the reverters.(ABSTRACT TRUNCATED AT 250 WORDS)