Examination of 19 serum biochemical and hematologic parameters in a group of white male runners, ranging in age from 23 to 47 years, just prior to and immediately after a 13-mile "mini-marathon," demonstrated a significant increase, by paired Student t-test, in mean values of: K+, BUN, creatinine, CK, LDH, AST (SGOT), alkaline phosphatase, bilirubin, uric acid and leukocyte counts. Prevailing environmental conditions were such as to produce no significant hemoconcentration. Using this group's statistics and this hospital laboratory's upper limits of normal, the percentage of values above two SDs are, for the resting state: K+ 7%, BUN 7%, creatinine 0%, CK 21%, LDH 21%, AST 0%, alkaline phosphatase 0%, bilirubin 7%, uric acid 7%, and leukocyte count 0%. Post-exertional values above normal limits are: K+ 7%, BUN 21%, creatinine 21%, CK 93%, LDH 86%, AST 0%, alkaline phosphatase 0%, bilirubin 14%, uric acid 36%, and leukocyte 71%. Consequently, abnormally high values for K+, BUN, creatinine, CK, LDH, bilirubin, uric acid, and leukocyte counts can often be expected in some patients who exercise heavily. The degree of the abnormality will depend on the level and length of exercise as well as the elapsed time between exercise and testing.