The effects of hyperinsulinaemia imposed on normal rats on the subsequent insulin-responsiveness in vivo of 2-deoxy-D-glucose uptake of white adipose tissue and of various muscle types were investigated. This was done by treating normal rats with insulin via osmotic minipumps, and by comparing them with saline-infused controls. Hyperinsulinaemia produced by prior insulin treatment resulted in a well-tolerated hypoglycaemia. At the end of the treatment, the glucose utilization index of individual tissues was determined by euglycaemic/hyperinsulinaemic clamps associated with the labelled 2-deoxy-D-glucose method. Prior insulin treatment resulted in increased insulin-responsiveness of the glucose utilization index of white adipose tissue, and in increased total lipogenesis in white adipose tissue and fat-pad weight. In contrast, prior insulin treatment resulted in a decreased glucose utilization index of several muscles. These opposite effects of hyperinsulinaemia on glucose utilization in white adipose tissue and muscles persisted when the hypoglycaemia-induced catecholamine output was prevented (adrenomedullectomy, propranolol treatment), as well as when hypoglycaemia was normalized by concomitant insulin treatment and glucose infusion. Insulin suppressed hepatic glucose production during the clamps in insulin-treated rats as in the respective controls, whereas total hepatic lipid synthesis and liver fat content were greater in rats treated with insulin than in controls. It is concluded that hyperinsulinaemia itself could be one of the driving forces responsible for producing increased glucose utilization by white adipose tissue, increased total lipid synthesis with fat accumulation in adipose tissue and the liver, together with an insulin-resistant state at the muscular level.