The distribution of a tachykinin (substance P) and vasoactive intestinal peptide (VIP) and the number and morphology of the large granular vesicles (LGV) in the myenteric plexus of the colons of mice were investigated. Six of the 12 young, male, Swiss mice studied had been inoculated with the Y strain of Trypanosoma cruzi 2 months previously whereas the others were uninfected controls. Substance P (SP) and VIP were localized by light microscopy, using an immunohistochemical method, and LGV were counted in sections studied by electron microscopy. There were far fewer LGV and less intensely staining varicose VIP- and SP-positive nerve fibres in the infected mice than in the controls. Denervation of the myenteric plexus may decrease the content of tachykinins (TK) and VIP in animals infected with T. cruzi. Such reduction in TK and VIP activity could be related to the disturbances in intestinal motility observed in the chronic phase of Chagas disease.