The effects of 7 successive days of prolonged jogging on aerobic performance and biochemical markers of muscle and red blood cell damage were examined in 10 moderately fit men, ages 27 ± 2 yr (mean±SE). The subjects jogged for 2 h per day at 78 ±4% of maximal heart rates and covered a total of 129 ± 2 km, nearly eight times their regular weekly training distance. At baseline, maximal oxygen uptake (V̇O2max) during treadmill tests averaged 3.45 ±0.24 L/min, or 44.7 ± 1.4 ml/kg/min. On follow-up tests 2d after the week of increased training, V̇O2max (3.56 ±0.17 L/min) and treadmill performance were not significantly improved. Body weight declined from 79.5 ± 4.6 kg to 77.8 ± 4.4 kg (p < 0.05) because of reduced body fatness (16.8±2.3% to 13.6± 1.7%, p < 0.05). Weight-adjusted V̇O2max increased to 46.4 ± 2.0 ml/kg/min (p < 0.05). However, heart rate and systolic blood pressure were not significantly changed at rest, or during submaximal and maximal treadmill exercise. Mean hemoglobin concentration at treadmill testing declined from 14.9 ±0.3 g/dl to 13.3 ±0.3 g/dl (p < 0.05). Leg muscle soreness, especially in the thigh region, persisted in all subjects after 3 d. Soreness was accompanied by chronic elevations (p < 0.05) in serum levels of myoglobin, creatine kinase (CK), lactic dehydrogenase (LD), aminoaspartate, and the isoenzymes CK-MB and LD1,2. Serum haptoglobin levels after jogging fell from 86±9 mg/dl to 60± 8 mg/dl (p < 0.05), suggestive of foot-strike hemolysis. It was concluded that leg muscle fibers and red blood cells were damaged by the unaccustomed high training distance. The reduction in hemoglobin concentration may have attenuated potential training effects on aerobic performance by decreasing the oxygen carrying capacity of the blood. The intensified training did not promote a short-term gain in cardiovascular fitness.