Plasma insulin and C-peptide levels in the fasting state and after a 2-h 75 g oral glucose tolerance test (OGTT) in a large number of healthy subjects are reported. 247 volunteers (134 males, 113 females), aged 13-69 years, who had a negative history of diabetes, no history of significant disease, normal physical examination, normal body weight, normal glucose tolerance, normal blood tests, and who were taking no drugs were studied. Results, mean +/- SEM (range): fasting glucose concentration = 4.64 +/- 0.03 mmol/l (3.10 - 6.10), 1-h glucose concentration = 5.23 +/- 0.10 mmol/l (2.20 - 9.90), 2-h glucose concentration = 4.11 +/- 0.06 mmol/l (2.00 - 6.80); fasting insulin level = 0.088 +/- 0.002 nmol/l (0.03 - 0.28), 1-h insulin level = 0.45 +/- 0.01 nmol/l (0.06 - 1.63), 2-h insulin level = 0.24 +/- 0.01 nmol/l (0.05 - 1.12); fasting C-peptide concentration = 0.60 +/- 0.01 nmol/l (0.14 - 1.34), 1-h C-peptide concentration = 2.17 +/- 0.05 (0.63 - 8.56), 2-h C-peptide concentration = 1.77 +/- 0.04 nmol/(0.35 - 5.74). Fasting insulin and fasting C-peptide concentrations correlated to post-glucose insulin and C-peptide concentrations, respectively. At each sampling-point insulin concentration correlated to C-peptide concentration. After glucose ingestion, both insulin and C-peptide plasma levels correlated significantly with the corresponding glucose levels. During fasting, C-peptide but no insulin level correlated to glucose level.(ABSTRACT TRUNCATED AT 250 WORDS)