Limited information is available on the healing of wounds in the mitral valve. In the present study, we have evaluated this process in young sheep. Histologic, histochemical and transmission electron microscopic studies were made of the healing of wounds produced surgically under cardiopulmonary bypass in the mitral valve (a 1 cm long radially oriented incision at the free edge of the anterior leaflet) of young sheep. All animals (n=11) developed severe mitral regurgitation, as demonstrated by hemodynamic, ultrasound and ventriculographic studies. At 4-5 weeks, the edges of the incision were only partially covered by endothelium and had small deposits of fibrin. Two types of granulation tissue were associated with wound healing. The first covered the cut edge and consisted of myxoid tissue that extended from the valvular spongiosa and fibrosa. The second type consisted of spindle-shaped cells that were arranged parallel to the surface and were surrounded by a slightly myxoid stroma. This granulation tissue resembled that caused by reactive fibrosis in regurgitant mitral valves. The connective tissue cells in granulation tissue developed increasing amounts of actin-like filaments as a function of time after operation. This differentiation resulted in the development of myofibroblasts. In contrast to the avascular nature of normal mitral leaflets, capillaries and arterioles first appeared in the middle portion of the leaflet at 8 weeks, and at the proximal edges of the wounds at 12 weeks. The healing of mitral valvular wounds is a slow process that requires 8 to 12 weeks for the formation of a dense collagenous scar at the edges and complete restoration of the endothelial lining. Progression of healing is associated with the phenotypic modulation of connective tissue cells from fibroblasts to myofibroblasts and neovascularization of the leaflets.