We demonstrated the possibility of a successful transplant of foetal rat small intestine into the peritoneal cavity of adult rats. Several combinations of rat strains with defined genetic differences were chosen. On the whole, in more than 66 percent of the performed transplantations a successful adequate growth of the transplants was noted. Oral application of Cyclosporin A (20 mg/kg/d) as immunosuppressive to prevent allograft rejection was necessary only in the combination across major genetic barriers. The preoperative insertion of a 7-0 non-absorbable thread into the lumen of the transplant helped to develop a tubular segment of small intestine which can be anastomosed to the intestinal continuity of the host in a second stage operation.