Endurance training increased VO2 max significantly from 71.6 +/- 1.3 to 81.5 +/- 1.7 ml . kg-1 . min-1 in female rats. Oxygen consumption and respiratory exchange ratios were observed in rats for 1 h at rest, running at 14.3 m . min-1 on a 1% grade (easy exercise), and running at 28.7 m. min-1 on a 15% grade (hard exercise). In hard exercise untrained rats had a higher respiratory exchange ratio (0.97 vs 0.90) and exercised at a higher percent VO2 max (92 vs 74%) than trained animals. Blood lactate was higher during hard exercise than during rest or easy exercise, and higher in untrained than in trained animals during exercise. Blood glucose was significantly higher in trained than in untrained animals during hard exercise, but other wise there were no differences between treatments or groups. These results suggest enhanced lipid oxidation and carbohydrate sparing in trained during prolonged exercise as the result of training. The improvement in whole-body VO2 max due to training (13.9%) was less than the increase in tissue respiratory capacity (50--100%) reported to accompany endurance training of rats. The improvement in VO2 max of rats as the result of training was of the same magnitude as the training response usually seen in humans.