Although it is known that exercise exerts a positive regulatory effect on collagen synthesis, the effects of endurance training on muscle endomysial connective tissue in man are not so well documented. To investigate this, a single muscle biopsy was collected from two groups of volunteers - endurance-trained sports participants and age-matched healthy untrained individuals. Endomysial staining intensity of types I, III and IV collagen was quantified by immunohistochemical staining and image analysis methods. Gelatinase activity in the endomysium was also quantified histochemically. Mean cycling VO2peak values of 53+/-6 (SD) and 32+/-8 mL/kg/min (P<0.01) were recorded for the trained and untrained groups, respectively. The staining intensity of types I, III and IV collagen and gelatinase activity in the trained group were not significantly different from the untrained group. However, when the data for all 11 volunteers were pooled, significant negative correlations were found for type III collagen staining intensity and capillary/fiber ratio; and for the relationship between type IV collagen staining intensity and VO2peak. These results suggest a negative association between aerobic capacity and the intensity of staining for types III and IV collagen in muscle endomysium.