A class of thiol compounds, mesna (sodium-2-mercaptoethanesulfonate), has been used to facilitate tissue dissection in surgical fields. To evaluate the technical feasibility and efficacy of chemically assisted endoscopic mechanical submucosal dissection (CAEMSD) by using mesna. An in vivo controlled blind trial by using porcine models. Nonsurvival study in an animal laboratory. Six pigs were studied. Mucosal targets of about 2 cm (estimated) for treatment were created with a circle of cauterized spots along the greater curvature of the gastric body and the antrum. A mesna solution or the control hydroxypropyl methylcellulose solution was submucosally injected. The target mucosa was mechanically isolated with balloon dissection and removed by circumferential incision with a hook-knife inserted into the submucosal space. Necropsy and mucosal specimens from both groups were examined by histology. En bloc resection of the target was achieved in all 8 attempts of the mesna group and in 7 of 8 attempts of the control group. Use of mesna significantly reduced tissue resistance to the initial balloon-catheter insertion into the submucosa and the technical difficulty of subsequent submucosal balloon dissection by using a subjective grading system (P < .05). There was no difference in histology between both groups. CAEMSD is a unique methodology to facilitate mucosal resection.