This study will provide information about the pharmacokinetics of mesalamine and its major metabolite in children being treated with Asacol for mildly to moderately active ulcerative colitis. It was designed to meet the expectations outlined in a Written Request issued by the FDA, as well as regulatory requirements to study the weight-based equivalent of 4.8 g/day. The age-appropriate dose formulation, the 400 mg tablet, will be used, and patients will be dosed every 12 hours, since this regimen is considered more convenient for pediatric patients and their parents.