Exercise intolerance is a problem in renal failure. Stationary cycle training during hemodialysis treatment is recommended as safe, effective, and practical, but requires compensations for both exercise and acute changes in uremia. Eight patients pedalled for 5 minutes, at 60% of VO2peak, at 0, 1, 2, and 3 hours of a hemodialysis treatment. Fluid removed, blood pressure, cardiac output, heart rate, O2 uptake, hematocrit, and arterial O2 content were measured. Mean arterial blood pressure, systemic vascular resistance, stroke volume, (a-v)O2 difference, and mixed-venous O2 content were calculated. Fluid removed was 1,356 mL/hr (P < 0.002 for each hour), but with no significant cardiovascular effects during the first 2 hours. At 3 hours, decreasing cardiac output, stroke volume, and mean arterial pressure all reached significance at rest (P < 0.05), and five of eight patients could not exercise. We conclude that the cardiovascular exercise response is superimposed on hemodynamic effects of dialysis and is adequately stable during the first 2 hours of treatment. After 2 hours, cardiovascular decompensation may preclude exercise.