Seven endurance-trained subjects were studied 12, 21, 56, and 84 days after cessation of training. Heart rate, ventilation, respiratory exchange ratio, and blood lactate concentration during submaximal exercise of the same absolute intensity increased (P < 0.05) progressively during the first 56 days of detraining, after which a stabilization occurred. These changes paralleled a 40% decline (P < 0.001) in mitochondrial enzyme activity levels and a 21% increase in total lactate dehydrogenase (LDH) activity (P < 0.05) in trained skeletal muscle. After 84 days of detraining, the experimental subjects' muscle mitochondrial enzyme levels were still 50% above, and LDH activity was 22% below, sedentary control levels. The blood lactate threshold of the detrained subjects occurred at higher absolute and relative (i.e., 75 ± 2% vs. 62 ± 3% of maximal O2 uptake) exercise intensities in the subjects after 84 days of detraining than in untrained controls (P < 0.05). Thus it appears that a portion of the adaptation to prolonged and intense endurance training that is responsible for the higher lactate threshold in the trained state persists for a long time (>85 days) after training is stopped.