Small bowel toxicity represents a major dose-limiting side effect of radiation treatment for many malignancies. We examined the effects of overexpressing human manganese superoxide dismutase (MnSOD) in the small intestine in mice to prevent radiation enteritis. Mice were treated with the human MnSOD gene delivered enterally using a nontoxic, replication-defective herpes simplex virus (HSV)-1-based vector. HSV vectors containing the human MnSOD transgene and green fluorescent protein (GFP) transgene, or GFP transgene alone, were constructed and injected intraluminally into a 2cm length of small intestine of C3H/HeNsd mice. Total body irradiation of 15 Gy was delivered to mice inoculated 24 hours earlier with either HSV-MnSOD (10(3) to 10(8) plaque-forming units), control HSV-GFP, or no vector. At 24 or 72 hours after irradiation, mice were killed and villi areas were measured from appropriate segments of the small intestine. Control irradiated mice showed a decreased villi area of 82% by day 3 after irradiation, whereas treatment of mice with HSV-MnSOD 10(8) plaque-forming units led to only a 16% decrease in villi area (P < 0.001) before radiation. Similar findings were seen on day 3 and were associated with a significant (P < 0.001) preservation of enteric protein content in HSV-MnSOD-treated mice. A dose-dependent effect of MnSOD in preventing radiation-induced small bowel injury was evident. These data demonstrate that overexpression of human MnSOD via a replication-defective herpes viral vector is an efficacious method of protecting the small intestine from ionizing radiation damage.