Three-hundred Balb/C mice were used to assess the effect of vasopressin on the response of the gastrointestinal tract to irradiation. Median lethal doses were determined for control and vasopressin treated animals in which the whole abdomen was irradiated. There was a significant difference in LD50/6 (882 +/- 27 for controls vs. 1051 +/- 23 for vasopressin-treated animals). In addition studies demonstrated a significant difference in the survival rate of jejunal crypts at 800 and 1,000 rads.