Polarization-microscopical measurements combined with silver-staining methods and morphometrical estimations (countings) were used to study the collagen fiber structures of the fibrosa of human atrioventricular valves as well as of the chordae tendineae of the right and left heart with regard to age and sex differences. Measurements and countings were performed on histological sections of the tricuspid valve, bicuspid valve and the chordae tendineae from a total of 87 subjects of both sexes, who were divided into three age groups (1st and 2nd, 3rd to 5th and > 6th decade) with six to seven persons in each group. Differences were found between the right and left atrioventricular valve and analogously between the chordae tendineae of the right and left heart in middle-aged persons with regard to the frequency-distribution H(Q) of the quotients of path differences and the proportion of silver stained collagen fibers as compared to unstained. These differences can be interpreted as an expression of different functional load (low-pressure vs. high-pressure system). Under higher pressure, the proportion of unsilvered fibers with a higher mean value of quotient (mainly assigned to type I collagen) at the expense of the silvered fibers (classified as type III collagen) becomes greater. With advancing age the total amount of collagenous fibers and the proportion of unsilvered fibers increases. Simultaneously, the mean values of the quotients (especially Q (unsilvered)) increase slightly. The study revealed sex differences for the tricuspid valve and the chordae tendineae of the right heart (low-pressure system) only.(ABSTRACT TRUNCATED AT 250 WORDS)