Lean, healthy normotensive sons of essential hypertensive parents (OHyp) have lower insulin sensitivity (SI) than sons of normotensive parents (ONorm). We have tried to find out whether this disturbance in insulin metabolism is related to altered body fat distribution, fuel metabolism, or both. 21 OHyp and 21 ONorm of similar age and body-mass index were investigated after fasting overnight. Body composition was assessed by dual-energy X-ray absorptiometry and fuel metabolism by indirect calorimetry and urinary nitrogen excretion. Plasma insulin and glucose concentrations were measured during the frequent sampling intravenous glucose tolerance test, and SI was calculated by the minimum model method. Systolic blood pressure and heart rate were slightly but not significantly higher in OHyp than ONorm but the groups did not differ in fasting plasma insulin or glucose concentrations, carbohydrate or lipid oxidation, lean and fat mass, bone mineral content, or distribution of body fat. By contrast, SI was significantly lower in OHyp than ONorm (8.2 [0.7] vs 13.4 [1.5] 10(-4) L mU-1 min-1, p < 0.01). Within the whole study population upper-body fat mass was positively correlated with fasting plasma insulin (r = 0.33, p < 0.03) and lipid oxidation was positively correlated with SI (r = 0.35, p < 0.04) and negatively correlated with subscapular/triceps skinfold thickness (r = -0.43, p < 0.01). Thus, impairment of SI precedes both the development of overt hypertension and gain or redistribution of body fat. Therefore, the concept that SI is low as a result of altered fat distribution has to be reconsidered, at least in young male offspring of hypertensive parents.