The expression of inducible nitric oxide synthase (iNOS) in two different murine wound models was investigated. Animals were subjected to either fullthickness linear skin incision with subcutaneous implantation of sterile polyvinyl alcohol sponges, or to 1.5 ؋ 1.5-cm dorsal skin excision. Previous work from this laboratory first described the expression of inducible nitric oxide synthase (iNOS) in a rat wound model. Using molecular and biochemical techniques, it was shown that iNOS is prominently expressed only early during wound repair, specifically the first 72 hours. Macrophages contributed the bulk of the iNOS protein to the wound, with the remainder found in polymorphonuclear leukocytes.1 Based on such findings, it was proposed that NO generated by iNOS could participate in the regulation of the inflammatory phase of wound healing.Other investigators have provided conflicting reports as to the role of iNOS-derived NO in the later stages of wound healing and the completion of the repair process. Yamasaki and colleagues 2 reported delayed healing in iNOS knockout (KO) mice that had undergone cutaneous excisional wounds, as well as in wild-type mice treated with an iNOS inhibitor. Additionally, they demonstrated that local adenoviral transfer of an iNOS expression vector normalized healing times in the iNOS-deficient animals. In contrast, Most and colleagues 3 found no difference between wild-type mice and iNOS KO mice in either the tensile strength of incisional wounds or the collagen content of subcutaneously implanted sterile polyvinyl alcohol (PVA) sponges.Thus, although iNOS expression seems to be unnecessary for the development of normal tensile strength or for the accumulation of collagen in incisional wounds, it has been shown to be required for the normal healing of excisional wounds. Attempting to reconcile these differences, experiments were performed to compare the expression of iNOS in incisional and excisional wounds in mice at the molecular and histological levels. Results demonstrate that the iNOS protein is present in sebaceous glands and skeletal muscle in both wound models. Furthermore, iNOS is strongly expressed in the inflammatory cellular infiltrate of excisional skin wounds. In contrast, iNOS is minimally expressed in the cellular infiltrate of sterile cutaneous incisional wounds and absent in the inflammatory cellular infiltrate that congregates in implanted sterile PVA sponges. Evidence to be presented demonstrates that extracellular bacterial colonization of the wound is required for the expression of iNOS, and that interferon (IFN)-␥ as well as functional lymphocytes are necessary for the full expression of iNOS in murine cutaneous wounds.