The post-exercise urine samples from 122 long-distance runners showed evident abnormalities upon microscopic examination in 95% of all subjects. Proteinuria, alone or with microscopic hematuria, was frequently found. Macroscopic hematuria was a rare occurrence. The urine samples collected in 30 runners before, immediately after the race, and 6, 12, 24, 36, and 48 h later showed a significant post-race decrease in the osmolarity and a significant increase in gamma-glutamyl transferase and N-acetyl-beta-glucosaminidase enzyme activity. Plasma renin activity and plasma aldosterone, determined before and after the race in nine runners, showed a significant increase in the post-race samples. The abnormal urinary findings disappeared in all cases within 24-36 h. It can be concluded that urinary abnormalities are very common after exercise. These abnormalities are most often of a "renal" origin, probably due to a temporary hemodynamic impairment, partially of glomerular but principally of tubular function.