For constant-load exercise of moderate intensity, oxygen uptake (VO2) increases monoexponentially, reaching a constant value within 3 min, i.e., steady state. However, at work rates associated with increased blood lactate, i.e., above the lactate threshold (LT), VO2 continues to increase slowly beyond 3 min; this delays or precludes steady state. We therefore correlated the characteristics of this slow phase of the VO2 kinetics with the increase in blood lactate during 6 randomized, constant-load cycle exercise tests in 6 normal men. One test was below and the others were at various levels above our gas exchange estimate of LT. The slow kinetic phase of VO2, characterized as the increase between the third and the sixth minute of exercise [delta VO2 (6-3)] or the third minute and termination of exercise [delta VO2 (term-3)], was linearly correlated with the blood lactate increase. The VO2 at termination of a given work rate above LT was greater than predicted from the sub-LT VO2 versus work-rate relationship. Work rates less than approximately 50% of the difference between VO2max and LT resulted in lactate and VO2 curves that reached recognizable asymptotes. At the higher work rates, both the lactate and VO2 curves continued to rise to the point of fatigue. We conclude that positive values for delta VO2 (6-3) and delta VO2(term-3) during constant-load exercise only occur at work rates above the LT, and the magnitudes of which are highly correlated with the increase in blood lactate.