By using immunocytochemistry (the whole mount stretch preparations and cryostat sections) and radioimmunoassay, the characteristics of distribution of calcitonin gene-related peptide (CGRP) in small intestine of rats and its change in acute intestinal radiation sickness are studied.The results show that all layers of the small intestine are rich in CGRP-immunoreactive(CGRP-I)nerves with a high density in the myenteric, submucosal and mucosal plexuses. Also it has a rather high density around the small blood vessels of the small intestine and intestinal crypts.Same CGRP-I neurons are seen in the myenteric and submucosal plexuses.In AIRS the level of CGRP in small intestine has a double-phase change which is lower at the 24th h followed by a higher level at 48th h, 72th h after the exposure. The results indicate that CGRP may be related to the regulation of motility, secretion,absorption sensation and regional blood flow of the gastrointestinal tract. CGRP is probably released under the stress of AIRS and participates in the mechanism of injury of AIRS through multiple ways, in particular, the influence on the regional blood flow and the increase of permeability of blood vessels.