The pattern of lactate increase and its relation to pyruvate and lactate-to-pyruvate (L/P) ratio were studied during exercise and early recovery in 10 normal subjects for incremental exercise on a cycle ergometer. Gas exchange was measured breath by breath. Lactate and pyruvate were measured by enzymatic techniques. Lactate and log lactate changed only slightly at low levels of O2 uptake (VO2) but both began to abruptly increase at approximately 40-55% of the maximal VO2. However, the point of abrupt increase in pyruvate occurred at higher work rates and the rate of increase was not as great as that for lactate. Thus L/P ratio increased at the same VO2 as the log lactate increase. Following the exercise, pyruvate continued to increase steeply for at least the first 5 recovery min, whereas at 2 min lactate increased only slightly or decreased. Thus arterial L/P ratio reversed its direction of change and decreased toward the resting value by 2 min of recovery. Lactate, as well as L/P ratios, decreased in all subjects by 5 min. This study demonstrates that lactate and pyruvate concentrations increase slightly at low levels of exercise without a change in L/P ratio until a threshold work rate at which lactate abruptly increases without pyruvate. The resulting increase in L/P ratio is progressive as work rate is incremented and abruptly reverses when exercise stops.