Eight well-trained middle and long distance male runners added to their regular training program a weekly 20-min treadmill run at a velocity calculated to elicit a blood lactate concentration of 4 mmol X 1-1. VO2 max, the running velocity eliciting 4 mmol X 1-1 blood lactate (VOBLA), and the activities of citrate synthase (CS), phosphofructokinase (PFK), lactate dehydrogenase (LDH) and LDH isozymes in the M. vastus lateralis were determined before and after 14 weeks of this training. Significant increases were observed in VOBLA and the relative fraction of heart-specific LDH, while the activity of PFK and the ratio of PFK/CS decreased after training. The change in VOBLA was negatively correlated to the mean rate of blood lactate accumulation during the last 15 min of the treadmill training runs, and positively correlated to the percentage of slow twitch fibers in the M. vastus lateralis. The data support the hypothesis that a steady state training intensity which approximates VOBLA will increase VOBLA, and will result in measureable local metabolic adaptations in the active skeletal muscles of well-trained runners without a significant change in maximal aerobic power. Muscle fiber type composition may be an indicator of the "trainability" of the musculature.