To investigate insulin insensitivity and its reversibility, we performed an insulin sensitivity test using the steady state plasma glucose (SSPG) method in 10 lean hypertensive subjects with normal glucose tolerance before and after treatment with alpha 1-blocker bunazosin, and 14 age body mass index-adjusted healthy control subjects. Steady state plasma glucose was significantly higher in the hypertensive subjects compared with the control group (182 +/- 10 mg/dL v 104 +/- 7, P < .01, mean +/- standard error of the mean (SEM). Steady state plasma glucose significantly decreased to 136 +/- 12 mg/dL (P < .01) after the treatment with alpha 1-blocker bunazosin, with a decrease of blood pressure. Hypertensive subjects had shown an increased area under the curve of glucose and insulin during the oral glucose tolerance test compared with normal controls. The glucose area decreased significantly, but the insulin area did not change after the treatment. There was no difference in plasma epinephrine, norepinephrine, and fractional excretion of Na between normal and hypertensive subjects both before and after treatment with bunazosin at basal and during insulin sensitivity tests (2 h). Serum total cholesterol level decreased and HDL cholesterol increased significantly after treatment with bunazosin. A significant correlation was observed between SSPG and blood pressure, but not between insulin level and blood pressure. The results indicate that insulin sensitivity is better related than hyperinsulinemia to hypertension and that this insensitivity is partially reversible by alpha 1-blocker, bunazosin.