Ten obese women with normal glucose tolerance, 10 obese patients with sulphonylurea-treated non-insulin-dependent diabetes (NIDD) and 11 patients with insulin-dependent diabetes (IDD; treated for 3-31 yr) took part in a 10-week programme of physical training in 50-min sessions, 2-3 times a week. As a result of training, maximal oxygen uptake increased significantly by about 18, 13 and 8% in the 3 groups, respectively, and citrate synthase in muscle increased significantly by 27-42%. The lipoprotein-lipase activity in muscle tissue (sampled by a needle-biopsy technique from the lateral vastus muscle) did not change. The number of capillaries/fibre in muscle tissue increased. This was accompanied by an increase in muscle fibre area, resulting in an unchanged number of capillaries/mm2. This may explain why the lipoprotein-lipase activity was unaltered. The latter activity in the group with IDD was lower than was predicted from the number of capillaries/mm2. This number was in fact larger than in the obese and NIDD groups. These results indicate that the increase in capillary density and lipoprotein-lipase activity that occurs in healthy young individuals as an effect of endurance training does not take place in obese, NIDD and IDD patients.