The metabolic and morphologic adaptation to physical training in skeletal muscle tissue of eleven middle-aged, physically untrained men was studied. Muscle biopsies were taken from the vastus lateralis before, after 8 weeks and after 6 months of physical training for analysis of metabolic and morphologic variables. Glucose tolerance test indicated increased insulin sensitivity after 6 months of physical training. The activities of glycogen phosphorylase, hexokinase and glucose-6-P-dehydrogenase were increased but other enzymes involved in glycogen turnover and glycolysis were unchanged after 6 months of physical traning. The activities of citrate synthase and cytochrome-c-oxidase, representing the oxidative capacity were significantly increased already after 8 weeks of physical training. The incorporation rate of palmitate-carbon into CO2 and triglycerides increased, and the incorporation rate of leucine-carbon into CO2 decreased with 6 months of physical training. The fiber diameter of both Type 1- and Type 2-fibers increased, while the mitochondrial volume increased predominantly in Type 2-fibers. Significant correlations were found between metabolic, physiologic and morphologic variables before and after physical training. The results indicate an increased oxidative capacity, mainly located to Type 2-fibers, and an increased utilization of fatty acids in response to this type of physical training.