The possibility that plasma levels of malonaldehyde (MDA) are altered by exercise has been examined. The presence of MDA has been recognized to reflect peroxidation of lipids resulting from reactions with free radicals. Maximal exercise, eliciting 100% of maximal oxygen consumption (VO2max) resulted in a 26% increase in plasma MDA (P less than 0.005). Short periods of intermittent exercise, the intensity of which was varied, indicated a correlation between lactate and MDA (r2 = 0.51) (p less than 0.001). Blood lactate concentrations increased throughout this exercise regimen. A significant decrease (10.3%) in plasma MDA occurred at 40% VO2max. At 70% VO2max plasma MDA was still below resting values, however the trend to an increase in MDA with exercise intensity was evident. At exhaustion, plasma MDA and lactate were significantly greater than at rest. These results suggest, that exhaustive maximal exercise induces free radical generation while short periods of submaximal exercise (i.e. less than 70% VO2max) may inhibit it and lipid peroxidation.