Inflammatory bowel disease (IBD) and small cell lymphoma (SCL) are chronic gastrointestinal disorders that negatively impact survival and quality of life in pet cats. In humans and other animals, including cats, changes in intestinal microorganisms are associated with chronic gastrointestinal diseases, including IBD and SCL. One way that microbes regulate intestinal health is by converting dietary tryptophan into chemicals called indoles. In humans and animal models of IBD, decreased microbial indole generation causes inflammation and intestinal damage. In preliminary studies, we identified decreased indoles in the feces and decreased tryptophan in the blood of cats with IBD and SCL. These findings are consistent with previous studies and suggest a prominent role for microbial indoles in IBD and SCL in cats. We aim to confirm our preliminary observations and identify the role of intestinal microbes in altered tryptophan metabolism in pet cats with IBD and SCL. We will quantify the concentrations of 7 different indoles in the feces and blood of cats with IBD and SCL and compare them to healthy cats. Using advanced DNA sequencing technology to identify intestinal microbes and their genes in the feces of cats with IBD, SCL, and healthy cats. By integrating these data using powerful statistical models, we expect to identify decreased indoles in association with changes in the abundance of specific intestinal microbes in cats with IBD and SCL. This study could inform the development of novel therapeutic strategies that could improve survival and quality of life in cats with IBD and SCL.