To determine the effect of the duration of incremental exercise on the point at which arterial blood lactate concentration (HLa) increases above the resting value (Iactate threshold : LT) and on the point at which HLa reaches a constant value of 4 mM (onset of blood lactate accumulation : OBLA), eight male students performed two different kinds of incremental exercise. In addition, the workloads corresponding to OBLA, which were obtained during both incremental exercises, were investigated for the validity of endurance training intensity. Incremental exercise bicycle tests consisted of 25 watts increase every minute (the 1-min test) and every 4 minutes (the 4-min test), respectively. VO₂ VCO₂, and V_Ewere measured by the Douglas bag method. Arterial blood samples were obtained from a catheter inserted into the radial artery. Blood HLa was analyzed by the enzymatic method. LT and OBLA were determined from the relationships between the HLa and VO₂ during the two kinds of incremental exercise tests. At the level of LT, OBLA, and maximal exercise, there was no significant difference in ventilatory and gas exchange parameters except for the workloads. Each subjects also performed two submaximal bicycle exercises for 15 minutes which intensities were corresponded to OBLA. During the submaximal exercise performed with the workload that had been determined with the 1-min test, HLa Increased gradually with duration and attained 8.8±1.2 mM after the end of exercise. On the other hand, when the submaximal exercise was performed with the workload which had been determined with the 4-min test, HLa remained at the predetermined value (4.0±0.4 mM). It was concluded that the lactate steady state condition was necessary to determine the endurance training intensity.