Six male subjects exercised on a bicycle ergometer 30 min with left leg and 30 min with right leg 3 times a week for 8 weeks. This training resulted in a 14.6% increase in VO2 max with two-leg exercise and a 23.1% increase with one-leg exercise. A significant decrease towards pretraining VO2 max was seen during the following 8 weeks of detraining. Muscle biopsy samples were obtained at rest from m. vastus lateralis before and after training and 4 and 8 weeks after training. During training the number of capillaries per mm2 and the number of capillaries per fiber increased about 20%. The number of capillaries around each fiber type (CA) increased 20--30%. The average area of each fibre type increased only about 5%. The fibre area per CA decreased by about 10%. During 8 weeks of detraining decreases were seen in the number of capillaries per fibre, CA and in fibre area, while fibre area per CA and number of capillaries per mm2 were almost unchanged at the end of the detraining period. Pronounced increases in activities of oxidative enzymes were observed after training, while only minor increases were seen in glycolytic enzyme activities. All enzyme activities decreased towards pre-training levels during detraining. The results indicate that the training-induced improvement in oxidative capacity and in muscle capillarization expressed as capillaries per fibre and CA disappears within 8 weeks after cessation of training. However, the fibre area per CA and number of capillaries per mm2 point at a favourable long term effect on the average diffusion distance between capillaries and muscle fibres.