The C‐peptide has recently been suggested to have beneficial effects in several organs and improve glycaemic control in human type I diabetes, while there were no such effects in healthy controls. The exact mechanisms behind these effects are, however, not clear. In an attempt to study the actions of C‐peptide on the microvasculature in normal rats during more controlled conditions, isolated rat hindquarters and kidneys were perfused with albumin solutions in order to obtain low basal concentrations of C‐peptide. In rat hindquarters, infusion of C‐peptide significantly increased the capillary filtration coefficients (CFC) from 0.035±0.002 to 0.044±0.002 mL min‐1100 g‐1mmHg‐1(P<0.001,n=9) and the permeability surface area product (PS) for vitamin B12from 3.48±0.29 to 4.02±0.37 mL min‐1100 g‐1(P<0.01, n=6). Addition of C‐peptide to the perfusate during infusion of sodium nitroprusside did not induce any additional alteration of CFC or PS. The vascular resistance was slightly decreased from 2.74±0.17 to 2.64±0.17 mmHg min 100 g mL‐1(P<0.01, n=9). These effects of C‐peptide are compatible with increases in capillary surface area without alteration of the permeabilityper se.In isolated rat kidneys perfused at low temperature (8 °C) prepared to inhibit all metabolic processes, C‐peptide induced no changes in glomerular filtration rate, total vascular resistance or fractional albumin clearance. Therefore, C‐peptide causes active vasodilation of the normothermic microvasculature and hence recruitment of capillaries. These findings support the previous observations in man that C‐p