Strenuous exercise induces profound changes in renal hemodynamics and the protein content of urine. Postexercise proteinuria seems to be directly related to the intensity of exercise, rather than to its duration. The pattern of proteins identified in urine collected after exercise is different from normal physiological proteinuria. Immunochemical techniques demonstrate that postexercise proteinuria is of mixed glomerular-tubular type when heavy exercise is involved. The origin of proteinuria after light exercise seems to be preponderantly of glomerular type. Clearance of individual plasma proteins suggests an increased glomerular permeability and a partial tubular-reabsorption inhibition of macromolecules.