The concentration time course of lactate, pyruvate, and excess lactate during 4 min of exercise and 30 min of recovery was studied in 20 patients with heart disease and in 8 normal subjects. The level of excess lactate accrued from the exercise was found to correlate well with the level of delta oxygen consumption in normal subjects ( r = .922). The cardiac output of the subjects with heart disease was considered in relation to the regression of cardiac output on oxygen consumption for normal subjects previously established by Donald (Clin. Sci. 14: 37–73, 1955). The patients with subnormal cardiac outputs during exercise had higher excess lactate values and lactate/pyruvate ratios than normal subjects and patients with normal cardiac outputs at similar levels of work. Normal subjects carrying out more severe work showed even higher concentration of excess lactate than did the patients with heart disease at lower work levels. The increase in excess lactate was maximal during the 2nd min of submaximal exercise. The rate of increase was progressively diminished during succeeding minutes. Note: (With the Assistance of James Carr and Wayne Vaughan) lactate; pyruvate; excess lactate Submitted on December 10, 1963