We examined blood flow redistribution during exercise and its significance on exercise intolerance in chronic heart failure. Sixty-three patients with chronic heart diseases underwent symptom-limited maximal multistage exercise using a supine ergometer. We measured oxygen intake (VO2) and cardiac index (CI) using Fick's principle and leg flow with the thermodilution method at rest and during exercise. Patients were categorized in 5 groups according to their VO2 max; i.e., control group (n = 12), having normal right-sided cardiac pressure during exercise; A group (n = 8), having an abnormal right-sided pressure elevation, but normal exercise tolerance VO2 max greater than 20 ml/min/kg; B group (n = 19) VO2 max 20-15; C group (n = 17) also 15-10; and D group (n = 7), VO2 less than 10 ml/min/kg. At maximal exercise, the CI max and leg flow max were similar between the control and A groups; whereas, they decreased in the order of groups B, C and D. The ratio of leg flow/CI increased by 5 times from rest to maximal exercise in all groups, although the values at rest and at maximal exercise were similar among all groups. The relationship between CI and leg flow during exercise was linear in each individual patient. The coefficient of this regression line was extremely high (r = 0.98 +/- 0.02). Therefore, we calculated each regression line, leg flow = (a).CI +/- (b), with the gradient (a) as an index of blood flow redistribution to working skeletal muscles.(ABSTRACT TRUNCATED AT 250 WORDS)