The study investigated the effect of training on lactate and H⁺ release from human skeletal muscle during one-legged knee-extensor exercise. Six subjects were tested after 7–8 wk of training (fifteen 1-min bouts at ∼150% of thigh maximal O₂ uptake per day). Blood samples, blood flow, and muscle biopsies were obtained during and after a 30-W exercise bout and an incremental test to exhaustion of both trained (T) and untrained (UT) legs. Blood flow was 16% higher in the T than in the UT leg. In the 30-W test, venous lactate and lactate release were lower in the T compared with the UT leg. In the incremental test, time to fatigue was 10.6 ± 0.7 and 8.2 ± 0.7 min, respectively, in the T and UT legs (P < 0.05). At exhaustion, venous blood lactate was 10.7 ± 0.4 and 8.0 ± 0.9 mmol/l in T and UT legs (P < 0.05), respectively, and lactate release was 19.4 ± 3.6 and 10.6 ± 2.0 mmol/min (P < 0.05). H⁺ release at exhaustion was higher in the T than in the UT leg. Muscle lactate content was 59.0 ± 15.1 and 96.5 ± 14.5 mmol/kg dry wt in the T and UT legs, and muscle pH was 6.82 ± 0.05 and 6.69 ± 0.04 in the T and UT legs (P = 0.06). The membrane contents of the monocarboxylate transporters MCT1 and MCT4 and the Na⁺/H⁺ exchanger were 115 ± 5 (P < 0.05), 111 ± 11, and 116 ± 6% (P < 0.05), respectively, in the T compared with the UT leg. The reason for the training-induced increase in peak lactate and H⁺ release during exercise is a combination of an increased density of the lactate and H⁺ transporting systems, an improved blood flow and blood flow distribution, and an increased systemic lactate and H⁺ clearance.