The activity of pyruvate dehydrogenase (PDH) was measured in samples of triceps muscle obtained in 15 males, before and after immobilization for 5 weeks and 5 months of strength training carried out in random order with cross-over of treatment. Although the total PDH activity was unchanged at 300 nmol min-1 g-1, the proportion in the active form increased after strength training (62 +/- 10.1%) and decreased after immobilization (12 +/- 2.3%) compared with control (36 +/- 3.4%). In six subjects measurements were repeated after 10 min maximal exercise (arm ergometer). The proportion of PDH in the active form increased least after immobilization (to 52 +/- 13.6%), compared with control (95 +/- 11.7%) and post-training (98 +/- 6.6%). After exercise muscle glycogen fell to the greatest extent and lactate rose the least in the post-training state, with opposite findings post-immobilization, suggesting that PDH activation contributes to the control of lactate formation in muscle during heavy exercise, and that the effects of training and immobilization are mediated at least in part through changes in the activation of this regulatory enzyme.