Muscle biopsies were taken from the vastus lateralis muscle of an elite power lifter during training and following a 7-month detraining period. The effects of detraining were investigated by combining ultrastructure, histochemistry, and pertinent metabolic data. Muscle fibers were classified ultrastructurally as fast-twitch and slow-twitch. Fast-twitch fibers were histochemically subdivided into a fast-twitch glycolytic (FG) and a fast-twitch oxidative glycolytic (FOG) in order to compare oxidative capacities between biopsy 1 and biopsy 2. The high intensity level of strength training prior to biopsy 1 caused an apparent hypertrophy of all fiber types. Detraining and weight loss resulted in a reversal of the training effect toward "control" values and adjustments in the oxygen delivery system. Atrophy occurred in all of the fiber types and altered the fiber composition of the muscle. A shifting of fibers classified as FG to the more oxidative FOG fibers caused a significant increase in the percent distribution of oxidative fibers (slow-twitch + FOG). The hypothesis is presented that the mitochondrial content (both number and form) of the detrained fiber remains constant (for a strength trained muscle), but as the volume of the fiber decreases the mitochondrial volume percent increases.