We have designed this investigation to determine whether naftidrofuryl has an effect upon metabolism in vivo in man. Five subjects were studied during and after steady exercise on a bicycle ergometer. On the first occasion they received naftidrofuryl (300 mg orally) 20-40 min before exercise; and on the second they exercised without the drug to provide control observations. Blood samples were taken for the estimation of glucose, pyruvate, lactate, glycerol, acetoacetate and 3-hydroxybutyrate. Exercise with naftidrofuryl caused a significantly greater rise in blood pyruvate concentration during the exercise and post-exercise period compared with the controls. Changes in glucose, lactate, glycerol and ketone-bodies were not significantly different from the control values. Lactate/pyruvate ratios were significantly reduced with naftidrofuryl during the post-exercise period. Exercise causes a rise in intracellular anaerobic metabolism with associated increases in blood lactate/pyruvate ratios. This is followed by enhanced oxidative capacity during the recovery period as adduced from falling lactate/pyruvate ratios. The greater decline in lactate/pyruvate ratios with naftidrofuryl during the post-exercise period is evidence that naftidrofuryl is able to enhance cellular oxidative activity in vivo in man. Further studies in clinical situations are therefore indicated.