An isolated perfused rat hindlimb preparation was used to study the impact of local muscle adaptations induced by endurance exercise training on muscle performance and peak muscle oxygen consumption. Rats were trained for 12-15 wk by a running program (30 m/min up a 15% grade for 1 h/day 5 days/wk) shown previously to increase muscle mitochondrial enzyme activity. Sedentary (n = 11) and trained (n = 11) hindlimbs of similar size were perfused with a similar inflow (12.1 ml/min) at a similar oxygen content (18.1 ml O2/100 ml blood). Tetanic contractions (100 ms at 100 Hz) at 4, 8, 15, 30, 45, and 60/min were elicited in consecutive order. Initial tension was better maintained by muscles of trained animals at all frequencies above 4 tetani/min (P less than 0.05). Oxygen consumption (mumol.min-1.g-1) increased similarly in both groups at the lower contraction frequencies but was greater (P less than 0.05) in the trained [3.52 +/- 0.32 (SE)] than in the sedentary (2.44 +/- 0.31) group at 60 tetani/min. The peak oxygen consumption of the trained group (3.93 +/- 0.27) was 20% greater (P less than 0.05) than that of the sedentary group (3.28 +/- 0.28) when peak values for each animal, irrespective of the contraction condition, are compared. Blood flows to the contracting muscle (approximately 100 ml.min-1.g-1) and, therefore, oxygen deliveries (mumol.min-1.g-1) were not different between sedentary (7.99 +/- 0.56) and trained groups (8.35 +/- 0.61). Thus the 20% higher peak oxygen consumption was achieved by a greater oxygen extraction.(ABSTRACT TRUNCATED AT 250 WORDS)