The acid glycosaminoglycans of human mitral valves are differentiated histochemically by the critical electrolyte concentrations of alcian blue. Age- dependent changes in the composition of acid mucopolysaccharides in the ground substances of valves are investigated by histophotometric measurements between the free valve edge and the line of closure. With respect to these values, the concentration of the total acid mucopolysaccharides in mitral valves grows with aging. The first increase is seen until to the 4th decade, a second one with the 5th decade reaching its maximum in old age. Sulfomucopolysaccharides show similar variations; increasing until the 4th decade, they are followed by a slight decrease and new increasing until old age. The concentration of more alcianophilic chondroitin sulfates, however, reaches a maximum in the 4th decade, falling off in later life. No acid mucopolysaccharides of higher sulfation grade are detected in mitral valve ground substances. Hyaluronic acid represents the main part of acid mucopolysaccharides in mitral valves. With advancing age the content of hyaluronic acid grows at a rate equal to that of the total acid mucopolysaccharides and reaches its maximum concentration in the 8th and 9th decade. The chondroitin sulfates represents 20–22% of the total acid mucopolysaccharides. This ratio does not change substantially during the aging proces