The life-spans of four series of NEDH rats were compared: (1) single control rats, (2) single rats irradiated with 1000 R, (3) control parabiont rats, (4) parabiont rats one of which had received 1000 R. The stress of radiation (1000 R) produced a median life shortening of 571 days in single rats. Parabiosis followed by irradiation (1000 R) of a member of the pair shortened life 132 days. Parabiosis alone produced a median life shortening of 62 days. Ninety percent of the older irradiated parabiont pairs died with one or more tumors present. Parabiosis largely protected animals from the gastrointestinal radiation syndrome.