To determine whether blood pressure is associated with 24-h urinary C-peptide excretion in subjects with varying degrees of glucose tolerance, we studied 247 Japanese men aged 30-69 who had never been treated with antihypertensive medications or with insulin. Plasma glucose and insulin responses during a 75-g oral glucose tolerance test, blood pressure and body mass index were obtained and urinary C-peptide excretion, in total and per kg body weight, were examined by 24-h urine collection. In monovariate analyses, urinary C-peptide excretion per kg body weight increased significantly as the blood pressure level rose (p < 0.05). After adjustment for age and body mass index by analysis of covariance, this relationship remained significant (p < 0.05), where adjusted mean values (+/- SEM) of urinary C-peptide per kg body weight were 1.56 +/- 0.05 microgram/24h/kg in the normotensive group and 2.04 +/- 0.17 microgram/24h/kg in the hypertensive (stage 2-4) group. When stratified simultaneously by glucose tolerance status and blood pressure level, adjusted mean values of urinary C-peptide per kg body weight were significantly higher in diabetic hypertensives than in diabetic normotensives. These results suggest that increase in 24-h urinary C-peptide excretion, i.e. 24-h insulin secretion, might contribute to an elevation of blood pressure both in normal and diabetic individuals.