Exercise induces profound changes in the renal hemodynamics and protein excretion. Strenuous exercise provokes a major fall of the renal plasma flow and a reduction of the glomerular filtration rate. Despite these changes, the filtration fraction doubles at maximal exercise preserving the transfer of metabolites or substances through the glomerulus. A higher production of vasopressin and aldosterone enhances the tubular processes of water and electrolyte reabsorption, stabilising therefore the homeostasis during exercise. Urea, uric acid and lactate reabsorption are also increased. Postexercise proteinuria is directly related to the intensity of exercise rather than to its duration. This transient state may be explained by an increased glomerular membrane permeability and a partial inhibition of tubular reabsorption of plasma proteins. Postexercise proteinuria appears to be age-dependent. Exercise has an additional effect on protein excretion in patients with nephropathies (diabetes, renal diseases, kidney transplants).