We examined the effect of pioglitazone, a thiazolidinedione derivative that increases insulin sensitivity without increasing insulin secretion, on the development and maintenance of hypertension in sucrose-fed SHR. Nine-week-old male SHR received 12% sucrose dissolved in tap water as drinking water. For 5 weeks, half of the rats were given regular rat chow, and the rest were fed with rat chow containing 0.03% pioglitazone. In week 6, blood glucose and plasma insulin levels were examined before and after oral glucose administration by gavage. Sucrose treatment elicited a significant elevation of systolic blood pressure 3 weeks after the beginning of treatment; pioglitazone treatment attenuated this elevation. The insulin resistance and hyperinsulinemia observed in sucrose-fed SHR were prevented by pioglitazone treatment. Pioglitazone treatment also significantly reduced the urinary excretion of catecholamines and plasma renin activity, both of which were significantly greater in sucrose-fed SHR than in control SHR. Along with improving insulin sensitivity, pioglitazone treatment also attenuated the development of hypertension in SHR fed the regular rat chow, but not in WKY rats. These results indicate that insulin resistance and hyperinsulinemia play an important role in the development of hypertension in SHR probably through the activation of the renin-angiotensin system and sympathetic nervous outflow. This study also shows that chronic sucrose treatment exacerbated the development of hypertension through these mechanisms, precipitating insulin resistance.