To detect changes with age in structure of myocardial proteins, swelling properties of human myocardium in solutions of HC1 and NaOH were investigated. Fragments of myocardium were incubated in solutions of various pH values and increases in weight were used as measures of swelling ability. Maximal swelling occurred at pH 0.9, 2.4 and 11.2. Greatest swelling, probably of the Donnan equilibrium type, occurred at pH 11.2 and this value was used for subsequent studies. Swelling decreases rapidly in the 1st years of life, then appears to remain constant until 35 years of age after which it decreases, to apparently remain constant at a low level. Collagen content, dry weight and amount of solids extracted during swelling do not vary with age. Heating myocardium causes a decrease in swelling ability. The possible role of thermal denaturation in myocardial aging and physiological implications of swelling changes are discussed.