Objective：To evaluate the protective effect of N-acetylcysteine(NAC) on the small intestine of male rats after X ray irradiation of the whole abdominal region.Methods: Sixty male Spraque-Dawley rats were divided in normal control group (n=12),irradiation group(n=12),and NAC groups(50,200,and 300 mg/kg,n=12).Irradiation injury was induced with X ray at a single dose of 1 000 cGy(source-to-skin distance 80 cm) for the abdominal regions after the animals had been anesthetized with sodium thiopental(40 mg/kg,i.p.).NAC was started 3 days before irradiation and administered for 3 more days after irradiation,and then rats were euthanized after 12 h fasting.The terminal ileum samples were collected for crypt survival assay and counting in ileal villi.The blood samples were collected for examination of superoxide dismutase(SOD) activity,and the contents of malondialdehyde(MDA) and glutathione hormone(GSH) in the small intestine.The plasma levels of D-lactate and diamine oxidase(DAO) were also measured.Results: The mucosal structure of the terminal ileum was damaged after irradiation.The plasma levels of D-lactate,DAO and the content of MDA were significantly increased;the activity of SOD and the content of GSH were significantly decreased(P<0.01).NAC at different dosages increased crypt survival rates and the number of ileal villi in the terminal ileum(P<0.05,P<0.01),enhanced the activities of SOD and content of GSH(P<0.05,P<0.01),and deceased the concentrations of D-lactate,DAO and MDA(P<0.05,P<0.01).Conclusion: NAC may protect the small intestine from irradiation-induced injury by protecting the intestinal mucosal barrier,eliminating oxygen free radicals,and maintaining the internal oxidative balance and the structure and function of intestinal mucous.