The contributions of local and systemic factors to the regulation of mucosal mast cells and globule leukocytes have been examined in the rat. Nippostrongylus brasiliensis has been used to provide a potent immunological stimulus for mucosal mast cell hyperplasia and the roles of intestinal and extraintestinal sensitization observed by comparison of the gut mast cell responses to larval and adult worm infestations. Systemic effects of adult worm infestations have been examined in isolated Thiry-Vella loops of intestine. It is concluded that the extraintestinal phase of larval infestation is not obligatory for a gut mast cell response and that mast cell hyperplasia and globule leukocyte formation are not dependent on direct contact with the parasite or its products. The dissemination of the mast cell response and the general significance of the results are discussed.