The relationship between the respiratory oxygen debt and excess lactate, change in lactate, and change in lactate/pyruvate ratio was examined in 48 experiments in 13 patients and in 13 normal volunteers. The subjects exercised for either 4- or 10-min periods. The recovery period varied from 17 to 45 min. Methods of analysis of lactate and pyruvate were colorimetric in 18 experiments, enzymatic in 20 experiments, and both in 10 experiments. The correlation coefficients between respiratory oxygen debt and excess lactate, delta lactate, and change in lactate/pyruvate ratio were 0.83, 0.87, and 0.85, respectively, in the colorimetric determinations. The enzymatic group showed somewhat poorer coefficients of correlation for these same variables: 0.76, 0.77, and 0.83, respectively. However, none of these relationships were one-to-one, and the correlation coefficients were poor for all of these variables in both series when they were subdivided into two groups based on whether the respiratory oxygen debt was greater or less than 2.2 liters/m2. excess lactate oxygen equivalents; change in lactate pyruvate; lactate/pyruvate ratio Submitted on June 15, 1964