To delineate the possible age-related differences in blood lactate response during exercise and its relations to endurance performance, 34 male runners (aged 21 to 69 years) performed an incremental treadmill running test. There were no significant differences in training distance and relative body fat among younger runners (YR), middle-aged runners (MR), and older runners (OR). The 5-km run time slowed with age, but was ranked at relatively the same level in each age group. OR had a 23% (P less than 0.001) and 12% (P less than 0.01) lower maximal oxygen uptake (VO2max) and a 22% (P less than 0.001) and 11% (P less than 0.001) slower 5-km run time than YR and MR, respectively. However, mean VO2 corresponding to 4 mM of blood lactate (OBLA VO2) was the same among the groups when expressed as %VO2max (YR; 84.3%, MR; 85.9%, OR; 85.9%). Significant correlations were found between OBLA VO2 (ml.kg-1.min-1) and 5-km run time in each group (YR; r = -0.648, P less than 0.05; MR; r = -0.658, P less than 0.01; OR; r = -0.680, P less than 0.05). These results suggest that OR attain a given blood lactate level at almost similar %VO2max to YR and MR and that OBLA VO2 in OR is useful for evaluating an endurance performance as well as in YR and in MR.