Exercise proteinuria is a transient condition often observed following heavy exercise in man. A general belief is that exercise proteinuria in man is partially due to the decline in renal blood flow (RBF) associated with exercise. The purpose of this study was to determine if exercise proteinuria occurs in the dog, an animal whose RBF is constant during exercise. Chronic bladder cannulas were surgically implanted in four female mongrel dogs. Urine from both kidneys -was collected prior to, during, and after heavy treadmill exercise (30 minutes duration, 5mph, and 15% grade). Urine protein concentration was determined by the biuret method. When compared to resting control urine, protein concentration during exercise increased by 47 percent (P < .01). Urine flow rate at rest, .17 ± .02 ml/min, decreased to .09 ± .01 ml/min (P < .01) during exercise, resulting in a small but not statistically significant decline in protein excretion. In twenty-six tests, when comparisons were made between resting and post-exercise periods, urine flow rate was identical, urine protein concentration was significantly higher (P = .05) and urine protein excretion was also significantly elevated (P < .01). These data indicate that exercise proteinuria may occur in the absence of changes in RBF.