In this study, we investigated the chronic in vivo effect of adiponectin on insulin sensitivity and glucose metabolism by overexpressing the adiponectin protein in male Wistar rats using intravenous administration of an adenovirus (Adv-Adipo). Virally infected liver secreted adiponectin as high and low molecular weight complexes. After 7 days of physiological or supraphysiological hyperadiponectinemia, the animals displayed enhanced insulin sensitivity during the glucose tolerance and insulin tolerance tests. Glucose clamp studies performed at submaximal and maximal insulin infusion rates (4 and 25 mU x kg(-1) x min(-1), respectively) also demonstrated increased insulin sensitivity in Adv-Adipo animals, with the insulin-stimulated glucose disposal rate being increased by 20-67%. In contrast, insulin's effect on the suppression of hepatic glucose output and plasma free fatty acid levels was not enhanced in Adv-Adipo rats compared with controls, suggesting that high levels of adiponectin expression in the liver may lead to a local desensitization. Consistent with the clamp data, the activation of AMP-activated protein kinase was significantly enhanced in skeletal muscle (by 50%) but not in liver. One interesting finding was that in male Wistar rats, both AdipoR1 and AdipoR2 expression levels were higher in skeletal muscle than in liver, as it is the case in humans. These results indicate that chronic adiponectin treatment enhances insulin sensitivity and could serve as a therapy for human insulin resistance.