Plasma citrate was determined in seven asthmatic children exposed to 6 min treadmill running before and after 6 weeks of physical training. Resting plasma citrate concentration increased from 116 +/- 9 mumol/l (mean +/- SE) before to 158 +/- 11 mumol/l after training (p less than 0.02). Plasma lactate increase during exercise was lowered by training (p less than 0.02); the individual change correlated inversely with the training-induced rise in resting plasma citrate concentration (r = -0.77, p less than 0.05). The postexercise percentage fall in peak expiratory flow rate decreased from 32 +/- 13% before to 15 +/- 8% after training (p less than 0.05). The results agree with the hypothesis that an increased citrate concentration of tissues, by inducing an inhibition of phosphofructokinase, may be of regulatory importance for the decreased lactate accumulation following endurance training in man. In addition, a beneficial effect of training on exercise-induced asthma is demonstrated.