We examined the distribution and functional integrity of mast cells in intestinal longitudinal muscle in rats sensitized by two previous infections with Trichinella spiralis. A segment of jejunum was excluded from the gut before infection, and the remainder of the gut was anastomosed. Few mast cells were seen in muscle of noninfected control rats except in the region of the jejunal anastomosis. In rats sensitized by T. spiralis infection, mast cells were increased in number in the jejunum and the number of mast cells followed an aboral gradient down the entire length of the gut in continuity. In addition, mast cells were present in muscle of the excluded segment of sensitized rats. All mast cells were stained red with safranin. Functional integrity was assessed by the ability of mast cells to induce contraction after degranulation by antigen. In muscle from sensitized rats, contraction was induced in each region after exposure to T. spiralis antigen but not Nippostrongylus brasiliensis antigen. Contraction was inhibited by the mast cell stabilizer doxantrazole and the 5-hydroxytryptamine (5-HT) antagonist cyproheptadine. When antigen-induced contraction was expressed as a percentage of the maximum response of the tissue to exogenous 5-HT, the magnitude of contraction decreased along an aboral gradient down the intestine and correlated well (r2 = 0.878) with mast cell numbers. These results suggest that the increase in connective tissue mast cells in gut muscle after T. spiralis infection involves both local and systemic mechanisms.