Angiotensin II (Ang II) is one of the most potent vasoconstrictors, and the first specific and orally available Ang II-receptor antagonist, losartan (MK-954, DuP-753), has now come into clinical use. The primary site of insulin resistance, as measured by the glucose clamp technique, is skeletal muscle. Losartan specifically blocks Ang II-induced vasoconstriction, namely causes vasodilation, and may thus increase glucose delivery to skeletal muscle. We used the euglycaemic hyper-insulinaemic glucose clamp technique to assess insulin sensitivity (glucose disposal rate, GDR) or insulin (I) sensitivity index (GDR/I). In 21-year-old men we found negative correlations between GDR/I and blood viscosity (r = -0.69), haematocrit (r = -0.65), fibrinogen (r = -0.50), cholesterol/HDL ratio (r = -0.45), triglycerides (r = -0.46), body mass index (r = -0.64), waist/hip ratio (r = -0.57), resting heart rate (r = -0.46) and diastolic blood pressure (DBP) (r = -0.43), and with DBP (r = -0.62) and plasma adrenaline (r = -0.36) during mental arithmetic stress. In the Losartan Severe Hypertension Study five patients with a record of DBP > or = 115 mm Hg were examined before and on losartan monotherapy for an average of 6 weeks. GDR increased 27% and plasma noradrenaline decreased 40% (P < 0.05 for both) during treatment with losartan. Calculated whole blood viscosity decreased on losartan (P = 0.04) and the changes in GDR correlated with the changes in viscosity (r = 0.89). These results suggest that losartan, possibly by a sympathicolytic effect, lowers blood viscosity, causes vasodilation, and improves insulin sensitivity in essential hypertension.