Renal functions were investigated in 29 marathon runners and in 20 runners in connection with 16-kilometre long-distance run. Body weight in runners decreased after marathon run in average of 1.3 +/- 0.5 kg and after 16-kilometre long-distance run in average of 1.4 +/- 0.4 kg. Blood pressure decreased after both runs. Total proteinuria and albuminuria significantly increased after both runs. The significant non-glomerular erythrocyturia was found in 9 runners after marathon run and in 3 runners after 16-km long-distance run. Total catalytic activity of serum creatine kinase, and its isoenzyme MB significantly increased after both runs. Increase of isoenzyme MB creatine kinase after runs was lower than 6% of total catalytic activity of creatine kinase. These increases were caused by rhabdomyolysis and were connected with myoglobinuria. Serum myoglobin significantly increased after marathon run and after 16-km run. Serum urea, creatinine, phosphorus and osmolality significantly increased after both runs. Calculated GFR significantly decreased after both runs. FE(Na), FE(Ca), FE(P), FE(OSM) and FE(H2O) significantly decreased after both runs. FE significantly increased after marathon run, but after 16-km run non-significantly decreased. Renal function abnormalities were caused by dehydration, microtraumas in extrarenal urinary tract, protein catabolism, decreased urinary excretion of osmotically active substances, rhabdomyolysis, activation of renin-angiotensin-aldosterone system and other factors. Renal function abnormalities in runners were already not present 2-6 days after marathon run and after 16-kilometre long-distance run and investigated parameters were in normal range or they did not significantly differ from the initial values.