A totally implantable prosthetic sphincter was tested on dog small bowel and on dog ileostomies. External pressure sufficient to completely restrain passage of intestinal contents caused some reduction in mucosal blood flow but no gross or microscopic injury to the intestinal wall. Long-term in vivo studies pointed out two major problems, infection around the foreign body and fibrotic reaction, leading to intestinal obstruction. The in vivo prosthetic sphincter functioned perfectly and caused no injury to the underlying intestine. This system has promise for future clinical applications.