Notes: Experimentally induced wounds in animal models are useful in gaining a better understanding of the cellular and molecular processes of wound healing and in the initial evaluation of the safety and effectiveness of potential therapeutic agents. However, studying delayed healing has proven difficult in animals, whose wounds heal within a few days. In this report, we describe a novel method for establishing mouse wounds that require up to more than three weeks for complete closure, and we show the validity of this model in Smad3 null mice, which are known to display accelerated healing. Full-thickness wounds, measuring 0.3 by 1.0 cm, were made down to fascia on the dorsal aspect of the mouse tail in Smad3 KO mice and control littermates, approximately 1 cm distal to the body of the animal. The wounds were left to heal by secondary intention and were assessed histologically by computerized planimetry for wound closure at various times after wounding. These wounds in wild-type mice displayed delayed healing, with full closure occurring between 14 and 25 days after wounding. Complete closure of similar wounds in Smad3 null mice healed 30% faster (p 〈 0.01). By immunostaining with ki67, a marker for proliferation, Smad3 null animals also showed increased proliferation of dermal wound cells. Cultured dermal fibroblasts from Smad3 null mice showed increased baseline DNA synthesis and, interestingly, enhanced response to TGF-β1. By Western blot analysis, Smad3 null mice fibroblasts showed a compensatory increase in MAPK phosphorylation in response to TGF-β1, suggesting that MAPK overcompensation together with loss of Smad3 may be involved in the modulation of faster healing. We conclude that this novel tail wounding model can be useful for studying delayed or prolonged wound closure.Experimentally induced wounds in animal models can be useful in gaining a better understanding of the cellular and molecular processes of wound healing. Such models have also proven themselves valuable in the initial evaluation of the safety and effectiveness of potential therapeutic agents targeted for chronic non-healing wounds. (Gottrup, Agren et al. 2000). However, no ideal animal model exists which reliably reproduces delayed healing. In the mouse, a mammal whose genome has been completely cloned and which is easily manipulated genetically, wounds normally heal within a few days, and with a great deal of contraction. (Morris, Wu et al. 1997; Gottrup, Agren et al. 2000) There are models utilizing either genetically altered and inbred mice with certain characteristics that cause delayed healing. (Carmeliet 1995) However, it would be useful to have wound healing models that are applicable to most wild type mice used as controls and which would have a large enough window of observation before healing occurs. In this report, we describe a novel method for studying delayed healing in mice. This method utilizes full-thickness wounds made down to fascia on the dorsal and mostly hairless aspect of the mouse tail. The wounds are left to heal by secondary intention and assessed histologically by computerized planimetry for wound closure at various times after wounding. In this first report, the validity of the model was determined by studying control littermates and Smad3 null mice, which have been shown to display accelerated healing. The results shown here suggest that this is a useful model for studying delayed healing in mice.