Body composition, maximal oxygen uptake, plasma lipids, glucose and lipid tolerance, and plasma insulin were examined in middle-aged, physically well-trained men in comparison with randomly selected men of the same age. The well-trained men were characterized by a small adipose tissue consisting of small fat cells, and probably by an increased muscle mass. They had an elevated maximal oxygen uptake. Fasting plasma lipids were low. Assimilation of 100 g glucose perorally was very rapid and occurred while insulin concentrations in plasma were much lower than in controls. Fasting plasma insulin values were also low. Intravenous lipid tolerance test showed a rapid removal rate of triglycerides. Analyses of glucose metabolism in vitro in muscle biopsies from these men showed an increased activity in several metabolic pathways. Succinic oxidase activity, as a marker of aerobic capacity as well as glycogen contents, was also increased. These results indicate that physical training is a potent factor for regulation of plasma insulin levels. It was suggested that qualitative and quatitative changes in muscle capacity to metabolize glucose are in some way involved in this regulation.