E pidermal TNF expression increases in response to cutaneous permeability barrier disruption and wound healing. TNF signaling is mediated by acid and neutral sphingomyelinases (A- and N-SMase), which generate ceramide, an important regulator of proliferation, differentiation, and apoptosis. In the epidermis, ceramide is known to be an integral part of the extracellular stratum corneum (SC) lipid bilayers that constitute the permeability barrier of the skin. We show here that topical application of TNF after experimental injury to the SC of hairless mice (hr^–/–) enhances barrier repair. In TNF receptor p55–deficient (TNF-R55–deficient) mice (hr^+/+), cutaneous barrier repair was delayed compared with wild-type (hr^+/+) or TNF-R75–deficient (hr^+/+) animals. After barrier disruption in hairless (hr^–/–) and wild-type (hr^+/+), but not in TNF-R55–deficient (hr^+/+) mice, the enzymatic activities of both A-SMase and N-SMase were significantly enhanced. Stimulation of SMase activities was accompanied by an increase in C₂₄-ceramide levels. Most A-SMase activity in hairless mice (hr^–/–) was found in the outer epidermal cell layers and colocalized in the lamellar bodies with A-SMase and sphingomyelin. Reduction of epidermal A-SMase activity by the inhibitor imipramine resulted in delayed permeability barrier repair after SC injury. Together, these results suggest that TNF-R55 signaling pathways contribute to cutaneous permeability barrier repair through SMase-mediated generation of ceramide.J. Clin. Invest. 104:1761–1770 (1999).