There is accumulating evidence for a purinergic humoral system involved in the control of cochlear function. Evidence of specific P2 purinoceptors on cochlear tissues implies a role for extracellular adenosine triphosphate (ATP) in the cochlea. To further this hypothesis a study was undertaken to determine if there was any specific source of purine compounds in cochlear tissues. Cochlear tissues (the sensory epithelium and lateral wall) from the guinea pig were incubated with the acridine derivative quinacrine dihydrochloride (5 x 10(-6) M in phosphate-buffered saline for 30 min at room temperature) which fluoresces on binding to high concentrations of ATP. Most cochlear tissues showed a diffuse green fluorescence slightly above the background level. However, a region of the marginal cells of the stria vascularis showed a specific punctate fluorescence. Optical sectioning of these cells by confocal microscopy revealed that the fluorescent structures in these marginal cells was confined to a region up to 10 microns from their endolymphatic surface. Similar cells studied by transmission electron microscopy showed membrane-bound vesicles located in the same region of the cell. These data imply that purine compounds are localized in discrete structures, perhaps vesicles, within the marginal cells which could serve as a source of extracellular ATP in the cochlea.