The gut system absorbs water, and thereby its injury leads to body fluid loss such as diarrhea. However, the detailed effects of gut injury on body sodium/water balance and blood pressure remain to be clarified. In the present study, we examined changes in body sodium/water balance and blood pressure in a mouse model of colitis. We induced a colitis model with 3% dextran sulfate sodium (DSS) by drinking water in male C57BL6J mice. 7 days after DSS administration, colitis mice decreased body weight but significantly increased relative total body sodium and water content per tissue dry weight. Colitis mice significantly decreased urine volume per water intake coupled with increased urea content in the renal medulla. On the other hand, DSS-induced colitis mice significantly decreased blood pressure measured by a radiotelemetry system. These findings suggest that in mice with colitis, enhanced urea-driven renal water reabsorption and a decrease in urine volume induce an increase in total body tissue water content. While colitis mice increased tissue water content, blood pressure was decreased, suggesting that diarrhea caused by colitis decreases blood volume and that tissue water and blood are not equilibrated. Colitis may cause abnormal body electrolyte/water balance at the tissue level, and it may be necessary to evaluate and correct the electrolyte-water balance.