PIP: An ultrastructure study was undertaken in an attempt to confirm the finding, obtained by light microscopy, that the new mesothelium arose by metaplasia of subperitoneal fibroblasts and not by transformation of peritoneal macrophages and to assess the role of detached mesothelial cells in peritoneal regeneration. A total of 80 adult male Sprague-Dawley rats weighing 220-250 grams was used. During the early stages of the healing process, macrophages were the predominant type of cell on the wound surface, but cells resembling primitive mesenchymal cells were occasionally observed. Gradual primitive mesenchymal cells and cells resembling proliferating fibroblasts became more numerous on the wound surface, while the number of macrophages decreased. By the 8th day following the operation a continuous layer of mesothelial cells was present on the wound surface. They resembled the underlying fibroblasts in nuclear and cytoplasmic characteristics, except that the mesothelial cells possessed microvilli and junctional complexes. There was no evidence in support of the theories that the new mesothelium arises from transformation of peritoneal macrophages or that is arises by seeding of mesothelial cells from adjacent peritoneal surfaces. The new mesothelium arises from subperitoneal connective tissues. As yet it has not been possible to determine conclusively whether these are primitive mesenchymal cells or fibroblasts.