We have studied the expression of alpha-smooth muscle actin, smooth muscle myosin, and desmin in granulation tissue during the healing of an open wound in the rat by means of electron microscopy and immunohistochemistry, at the light and electron microscopic levels, using specific antibodies directed against these proteins. Important amounts of the three antigens were always expressed in pericytic and/or smooth muscle cells of neoformed small vessels. In fibroblastic cells, microfilaments were absent 4 days after wounding but accumulated gradually, starting from the 6th up to the 15th day; at this time, they were evident in about 70% of fibroblastic cells (myofibroblasts). They then regressed progressively, and on the 30th day, microfilaments were no longer present in scar fibroblasts. alpha-Smooth muscle actin, but not smooth muscle myosin and desmin, was always present in microfilament bundles of myofibroblasts. The staining intensity was progressive from the 6th to the 15th day and decreased thereafter; no staining was seen at the 30th day. Between the 20th and the 25th day, many apoptotic figures were seen in fibroblastic and endothelial cells, suggesting that apoptosis is the mechanism of their disappearance. We conclude that myofibroblasts develop gradually from granulation tissue fibroblasts and temporarily express a marker of smooth muscle differentiation. These results may be relevant for the understanding of the mechanisms of normal and pathologic wound healing.