The histochemistry of intestinal (IMC) and connective tissue mast cells (CTMC) in the normal rat is compared. Acid mucopolysaccharide appears to be less strongly sulfated and the granule content of monoamines is lower in IMC. After infection with the intestinal helminth, Nippostrongylus brasiliensis, the mucosal content of IMC is altered. During the early phase of immunologic expulsion of this parasite (self-cure) the IMC proliferate and differentiate; the histochemical properties of the granules appear to reflect this process. Very large numbers of mature IMC are found in the mucosa during the later, rapid phase of worm expulsion. Both acid mucopolysaccharide and monoamines are depleted from the granules of these cells; this is consistent with previous studies suggesting that IMC discharge occurs at this stage. These findings lend further support to the hypothesis that the biogenic products of IMC discharge are responsible for the increase in mucosal permeability which occurs during self-cure and which may facilitate the rapid translocation of antiworm antibody into the intestinal lumen.