The effects of 15 days of detraining and 15 days of retraining were studied in 6 well-trained runners. Detraining resulted in significant decreases in the mean activities of succinate dehydrogenase (SDH) and lactate dehydrogenase (LDH) of 24% and 13% respectively, but no significant increases in these enzymes activities occured with retraining. Maximal oxygen uptake (VO2 max) decreased by 4% with detraining (p less than 0.05), and increased by a similar amount with retraining. Performance time in an intense submaximal run decreased by 25% (p less than 0.05) with inactivity, but still averaged 9% below the initial level after retraining. Maximal heart rate and peak heart rate during the performance run were higher after detraining by 4 and 9 beats per min, respectively (p less than 0.05). With retraining, these heart rate values were decreased by 7 and 9 beats per min (p less than 0.05). Blood lactate concentrations after the VO2 max and performance run were approximately 20% lower after detraining and retraining (p less than 0.05). Muscle fibre areas for three subjects tended to be larger in biopsy samples taken after detraining and retraining. These data suggest that even short periods of detraining result in significant changes in indices of physiological capacity and function in subjects near their upper limit of adaptation, and that a longer period of retraining is necessary for muscle to re-adapt to its original trained state.