Recent studies have demonstrated that activation of the 5-HT4 receptors in the colonic mucosa can have healing and protective actions in experimental models of colitis. These actions include increased mucus secretion, increased epithelial proliferation, and enhanced epithelial migration. Since these studies involved chemically induced models of colitis, the current investigation was conducted to test whether a protective action of 5-HT4 receptor stimulation could be detected in Interleukin-10 knockout (IL-10 KO), which develop colitis spontaneously due to the absence of the anti-inflammatory cytokine, interleukin-10. Upon weaning, the IL-10 knockout mice were separated into two groups: an agonist group and a vehicle control group. The agonist group received 1 mg/kg tegaserod in a vehicle consisting of 0.9% saline each day by enema of dimethyl sulfoxide (DMSO) in saline each day, while the control group received daily enemas of vehicle over the course of 21 days. Several outcome measures were used to assess the effectiveness of the treatment. To evaluate the severity of colitis, disease activity index was monitored, and histologic damage was blindly scored. Administration of tegaserod by enema to the IL-10 KO mice had a significant protective effect on the treated mice. The disease activity index (DAI) of agonist treated mice was significantly better than that of vehicle treated mice over time (p<0.001; 2-way ANOVA). Mice treated with vehicle had a more significant decline in health over time versus the agonists, with more blood present in feces and a looser/diarrhea-like consistency in stool. The histological damage score (HDS) was also improved by 5-HT4 agonist treatment (p<0.05, t-test). Sections of vehicle treated colons showed significantly greater damage, including epithelial erosions, the presence of polymorphonuclear cells, and abnormal crypt architecture (cryptitis), than those treated with the 5-HT4 receptor agonist tegaserod. During the 21-day course of the current investigation, there was no difference in the survival data between the two groups. These data, when taken together, suggest that administration of the 5-HT4 receptor agonist tegaserod via enema to IL-10 KO mice has a greater healing and protective effect than seen in the IL-10 KO mice that received vehicle. We proposed to test the hypothesis that treatment of IL-10 knockout colitis with a 5-HT4 receptor agonist will attenuate the development of colitis and have healing and protective effects in the colons of the treated mice.