Type II diabetes is a disease characterized by chronic hyperglycaemia and abnormalities in lipid metabolism that affects approximately 5% of the population in the Western World. Caveolae are invaginations of the plasma membrane, described as 25-150 nm omega shaped structures, which are enriched in cholesterol, sphingolipids and the constituent protein caveolin. Caveolae have been shown to be involved in signal transduction, uptake over the plasma membrane and intracellular transport. By electron microscopy studies of cell membranes and biochemical analyses of isolated caveolae, we report that in rat adipocytes glucose transporter GLUT4 was translocated to caveolae in response to insulin. Insulin stimulation increased the amount of GLUT4 in the plasma membrane, but the ratio between GLUT4 in the planar and caveolae membrane remained constant. These findings indicate that caveolae are the locales for glucose uptake in the cell. We also report that the insulin receptor, independently of insulin stimulation, was localised in caveolae in human adipocytes. In these cells depletion of cholesterol destroyed the caveolae structure and the adipocytes became insulin resistant. Cholesterol depletion did not affect the insulinstimulated autophosphorylation of the insulin receptor nor the phosphorylation of the downstream IRS1. Further signalling to metabolic control or mitogenic control was inhibited, however. With transmission electron-, scanning electron- and fluorescence-microscopic techniques, we studied the ultrastructure and distribution of caveolae in the rat adipocyte. We found that caveolae can be divided into two subpopulations, small (<50 nm) and large (50-150 nm). The large caveolae are connected to the extracellular space via narrow necks and the orifices of caveolae were herein shown in primary adipocytes for the first time. Caveolin is located in the membrane proximal part of the small caveolae and to the neck in the large caveolae. The insulin receptor substrate IRS 1 was shown to be localized to caveolae in human adipocytes and to colocalize with the insulin receptor. In rat adipocytes, however, IRS1 was not localized to the plasma membrane in the absence of insulin stimulation. By transfection of rat adipocytes with human IRS1 we found that human IRS1 bound to the plasma membrane in the rat adipocyte, whereas the endogenous rat IRS1 did not. Taken together, caveolae seem to be closely involved in regulation of insulin action in the adipocyte.