The aim of this study was to determine whether insulin sensitivity measured by the euglycaemic insulin clamp technique is lower in patients with primary hypertension than in matched healthy control subjects, and whether this sensitivity was affected after 12 weeks of antihypertensive treatment with the alpha 1-adrenoceptor blocking drug prazosin. Twelve moderately obese normoglycaemic patients (four men), with hypertension not previously treated with pharmacological agents and diastolic blood pressure above 100 mm Hg, and 12 healthy matched control subjects participated. Supine blood pressure decreased 12/5 mm Hg (p less than 0.01) and standing blood pressure 14/9 mm Hg (p = 0.001) during prazosin treatment (mean dosage 5.3 +/- 1.6 mg/day (SD]. During euglycaemic insulin clamp studies the control subjects showed a higher mean glucose uptake than the untreated hypertensive patients (7.5 +/- 1.0 and 5.8 +/- 1.9 mg.kg b.w.-1.min-1, respectively, p less than 0.01). During prazosin treatment there was no significant difference between the hypertensive patients and the control subjects in this respect (6.6 +/- 2.8 and 7.5 +/- 1.0, respectively, p = 0.21). During prazosin treatment, however, the disappearance rate of glucose decreased during the intravenous glucose tolerance test (from 1.7 +/- 0.9 to 1.3 +/- 0.6, p less than 0.02) and the area under the glucose concentration-time curve decreased by 38% (from 473 +/- 119 to 294 +/- 99, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)