The objective of this work was to study the collagen tissue of the human heart muscle as a function of age. The types of collagen, their disposition, as well as the density of collagen tissue and diameter of collagen fibrils were examined. Pieces of the ventricular wall from 12 human hearts, six from young individuals and six from aged individuals were studied by the Picrosirius-polarization method and by transmission electron microscopy (TEM). The results obtained showed the presence of two types of collagen fibers in the ventricular walls - thin, weakly birefringent, greenish fibers (collagen type III) and thick, yellow or red, strongly, birefringent fibers (collagen type I), both in the endomysium and perimysium. In the hearts obtained from old subjects, there were no significant differences in the arrangement of the collagen fibers in relation to the hearts obtained from young subjects. Measurements of collagen content in myocardial tissue suggest that both perimysial and endomysial collagen type I fibers increase in number and thickness in the old. These histochemical results obtained coincided with the electron microscopic observations in showing increase in the number of collagen fibrils with large diameter in the old hearts. These ultrastructural and histochemical characteristics of collagen may provide insights important to assessing the pathogenesis of the cardiac lesions of several cardiopathies in the aged heart.