Hyperinsulinaemia may play a causal role in the development of hypertension in obese hypertensives. However, experimental evidence supporting this statement is inappropriate. The main purpose of this study was to evaluate the chronic effects of insulin administration on blood pressure, total-body glucose metabolism and urinary catecholamine excretion. After 10 weeks of insulin injection blood pressure was substantially increased in insulin- treated animals compared to those treated with saline (125±2 vs 108±2 mm Hg, pcO.OOl). There were no differences in glycaemia, plasma triglyceride levels and free fatty acid levels between these two groups. Plasma level of corticosterone was increased in both insulin-treated and saline-treated rats as compared to untreated animals suggesting that the level of stress was similar in both injected groups. The urinary excretion of norepinephrine and dopamine was increased in the insulin-injected group by about 120 % and 310 %, respectively. Our data clearly indicate that long-term insulin administration increased blood pressure but the underlying mechanisms remain to be elucidated.