Nine normal young male students were studied during 2 days of relative rest, during 2 days of physical training and again during the succeeding 2 days of relative rest. 24-hour urine collections showed that the creatinine, creatine, uric acid, urea, calcium and magnesium excretion were similar during the resting and exercise days. The 24-hour urinary excretion of sodium and potassium was decreased during the exercise days, while the aldosterone excretion was increased. The hemoglobin concentration, hematocrit and red cell counts were decreased 14, as well as 42 h after exercise. The serum uric acid, creatine phosphokinase-MM (skeletal muscles) subfraction, glutamic oxalacetic transaminase and myoglobin levels were increased 14 h after exercise, but returned to baseline 42 h after this type of exercise. The mechanisms of these alterations were discussed and the data show that one should take into account previous exercise when interpreting the results of certain of these tests.