1. Blood antioxidants were measured in venous blood samples from 20 runners and six sedentary individuals. All subjects were male, between 20 and 40 years old, and in steady state with respect to body weight and physical activity patterns. Dietary analysis was undertaken using a 7-day weighed food intake. Correlations were sought between antioxidants in blood and (1) weekly training distance and (2) maximum oxygen uptake. In addition, 12 runners could be classified into two groups undertaking either low (range 16-43 km, n = 6) or high (80-147 km, n = 6) weekly training. 2. Body weight (range 55.3-90.0 kg) and percentage body fat (range 7-19%) both showed negative correlations with the weekly training distance (both P less than 0.001). Energy intake and maximum oxygen uptake were both correlated with the weekly training distance (both P less than 0.001). 3. Plasma creatine kinase activity, an indicator of muscle damage, was significantly correlated with the weekly training distance (P less than 0.01), whereas the plasma concentration of thiobarbituric acid-reactive substances, an indicator of free-radical-mediated lipid peroxidation, decreased with increased maximum oxygen uptake (P less than 0.01). 4. Erythrocyte alpha-tocopherol content was greater in the two running groups (P less than 0.05) compared with the sedentary group, and lymphocyte ascorbic acid concentration was significantly elevated in the high-training group (P less than 0.01) compared with the low-training group. 5. Erythrocyte activities of the antioxidant enzymes, glutathione peroxidase and catalase, were significantly and positively correlated with the weekly training distance (P less than 0.01 and P less than 0.05, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)