Effects of normal strength exercise on leukocyte accumulation were examined in 10 well-trained male subjects (27.2 +/- 2.7 yr). The workout, consisting of five maximal sets of three repetitions of leg press exercise and five maximal sets of six repetitions of knee extension exercise, was performed with the dominant leg, and the other leg served as control. Repeated maximal isokinetic knee extensions at 60 degrees /s were performed to evaluate neuromuscular fatigue and recovery after the workout. Accumulation of leukocytes was assessed with 99mTc-labeled cells, and repeated images of the thighs were taken 1-24 h after the workout. Maximal force-generating capacity in the exercised leg was reduced by 17 +/- 2% (P < 0.01) after the workout. The course of recovery followed a biphasic pattern characterized by halted recovery 10-23 h after exercise. The presence of leukocytes was approximately 10% higher in the exercised than in the control thigh 10 h after exercise (P < 0.05). This difference increased to approximately 15% at 20 h after exercise (P < 0.05). The retarded recovery of maximal force-generating capacity 10-20 h after exercise, together with a significant infiltration of leukocytes in exercised muscle during the same time interval, shows a temporal relation between leukocyte infiltration and impaired recovery.