Jejunal longitudinal muscle from Trichinella-infected Sprague-Dawley rats generates increased tension in vitro 6 days after infection. To investigate the extent to which this increase is dependent on T lymphocytes, smooth muscle contraction was examined in athymic rats (rnu/rnu) and in their euthymic (rnu/+) littermates 6 days after infection. In both thymus-bearing and athymic rats, the highest concentrations of Trichinella were found in the jejunum where mucosal myeloperoxidase activity was increased. Muscle from infected euthymic rats generated more tension to carbachol or 5-hydroxytryptamine than that of noninfected controls. In contrast, there was no difference between the responses of muscle from infected and control athymic animals. However, when athymic rats were reconstituted with splenic mononuclear cells, increased tension to carbachol or 5-hydroxytryptamine was observed after infection. T-lymphocyte activity was demonstrated by in vitro assays in euthymic or reconstituted rats but not in athymic rats. It is concluded that some changes in intestinal smooth muscle function following Trichinella spiralis infection in the rat are T lymphocyte dependent. These results support the concept of immunomodulation of intestinal smooth muscle function.