Intestinal intraepithelial lymphocytes (IEL), one of the major effector components in the mucosal immune system, are phenotypically and functionally distinct from thymic and peripheral T cells. To investigate the effect of repeated stress on the number and function of IEL, we exposed male C3H/HeN mice to mild electric foot shock for 30 min/day for 5 consecutive days. Immediately after the final foot shock stress, the blood, spleen, thymus and small intestine of each of the mice were obtained. As a functional measure, we evaluated interferon (IFN)- γ production by IEL, since IFN-γ is a key immunomodulating cytokine in mucosal immune responses. Serum corticosterone level was elevated immediately after foot shock stress. There were no significant changes in the number of whole IEL and CD3⁺ IEL subsets after the stress. In contrast, the stress led to a significant decrease in the total number of thymocytes, particularly the reduction in the number of CD4⁺CD8⁺ thymocytes. Thymocytes expressed the highest level of intracellular glucocorticoid receptor (GR), followed by splenocytes and IEL. The foot shock stress induced a marked suppression of IFN-γ production by IEL, when stimulated with immobilized anti-CD3 monoclonal antibody. Furthermore, corticosterone suppressed the IFN-γ production by cultured IEL, which was prevented by Mifepristone (RU486), a GR antagonist. In summary, repeated foot shock stress did not alter the numbers of IEL and CD3⁺ IEL subsets, but suppressed IFN-γ production by IEL, which was probably mediated by the elevated corticosterone. We therefore propose that stress influences host defense by suppressing the production of IFN-γ in IEL.