EXERCISE proteinuria was first reported1 in 1878, and since then increased excretion of protein in the urine during and after exercise has been extensively studied. The physiological mechanisms involved, however, remain unknown. We have found that not only did the activity of kallikrein in urine increase after exercise, but that intravenous administration of urinary kallikrein could induce experimental transient proteinuria in rabbits2. It therefore seemed to be relevant to the causes of exercise proteinuria to examine the mechanisms of proteinuria induced by urinary kallikrein.