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During the normal cyclic operation of the aortic valve its leaflets are subjected to continual bending, shearing, and membrane stresses. In vivo studies of marked leaflets have shown that the bending stresses are greatest where leaflets are attached to the aortic wall. Pressure stresses during diastole also appear to be high in this region. The internal shearing stresses are difficult to predict because of the semifluid nature of the tissue in the attachment zone. In the present study a model of the attachment region incorporated measurements of leaflet motion from dogs in vivo and from histological sections. From these measurements bending and membrane stresses were derived in order to estimate the total stresses. In this region the total stresses in systole were negligible because membrane stresses are essentially zero, but those in diastole ranged from 76 to 95 g.mm-2 in the circumferential direction and from 37 to 44 g.mm-2 in the radial direction. The calculated stress suggests that excessive tissue wear and valve failure could occur in the absence of tissue replacement. From radioautographic studies of rat valves, proteins and complex sugars of the valve connective tissues were found to be regularly replaced in patterns predictable from the level of stresses.