In order to evaluate whether borderline hypertension might be associated with hyperinsulinaemia, twenty non-obese borderline hypertensives and twenty matched normotensives underwent a standard oral glucose tolerance test and 24-h ambulatory blood pressure monitoring. Blood pressure, plasma glucose and insulin were measured at fasting and 15, 30, 60, 120 and 180 minutes after glucose load. Fasting plasma insulin was significantly higher in borderline hypertensives in comparison to normotensives (16.6 +/- 6.9 vs 12.4 +/- 4.2 mU/l; P < 0.05). Plasma insulin response estimated by the positive incremental area under the curve did not differ significantly between two groups but borderline hypertensives showed a larger interindividual difference. Decrease of systolic blood pressure after glucose load was significantly greater in borderline hypertensive subjects. Furthermore, blood pressure and plasma insulin relationship was different in borderline hypertensives compared to normotensives.