The effect of different speeds of incline running (16-48 m/min) on changes in glycogen, lactate, and alanine concentrations in different skeletal muscle fiber types and in blood were investigated in nontrained adult female rats during 5-min bouts of exercise. Glycogen depletion occurred extensively (25-86%) in fast-oxidative-glycogenolytic (FOG) and in slow-oxidative (SO) fibers over the range of speeds studied, whereas it occurred primarily (76%) at 48 m/min in the fast-glycogenolytic (FG) fiber type. No marked increases in lactate over resting control levels occurred in the FOG fibers. However, compared to resting levels, lactate increased over fourfold in FG muscle during running at 48 m/min and blood lactate concentrations paralleled these changes. No changes occurred in alanine concentration in FOG muscle and in blood under these experimental conditions, whereas there were approximately 70% increases in FG and SO fibers during moderate-to-heavy exercise. These findings suggest that elevations in blood lactate of rodents primarily result from lactate production in the FG fiber type.