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The G-protein subunit a-gustducin is a marker of chemoreceptive cells. In the present study, we examined the immunohistochemical localization of a-gustducin in rat airway epithelium both by light and electron microscopy. a-Gustducin immunoreactivity was found in solitary cells that presented ultrastructural features of chemoreceptor cells, i.e. flask-shaped or pear-shaped, with an apical process with thin microvilli protruding into the lumen. The immunostaining was mainly concentrated in the apical process and along the basolateral cell surface. To investigate whether a-gustducin-immunoreactive cells represented a distinct cell subset in rat airways, we performed double-label immunocytochemistry with antibodies to protein gene groduct (PGP) 9.5, a marker of neuroendocrine cells, and to phospholipase C beta2 (PLCß2), a component of the bitter signalling pathway. a-Gustducin-immunoreactive cells were present in a subset of PGP-9.5-immunoreactive elements, although not all a-gustducin-positive cells expressed PGP 9.5 labelling. In addition, a subset of a-gustducin-expressing cells colocalized PLCß2. This work thus demonstrates that solitary a-gustducin-immunoreactive cells exist throughout the airways and represent a specialized cell type with morphological and immunohistochemical characteristics of chemoreceptor cells. [ABSTRACT FROM AUTHOR]