To examine the cytochemical features of primary vestibular afferent neurons, we examined the immunocytochemical distribution of substance P(SP) and neurofilament proteins (NFP) in the guinea pig vestibular endorgans and ganglion. 84.9% of vestibular ganglion cells showed SP-like immunoreactivity. SP immunoreactive cells were small in diameter (25.4 microns), and randomly distributed in the ganglia. NFP immunoreactive cells (34.2%) were relatively large in diameter (31.1 microns). By double-staining immunocytochemistry it was possible to distinguish three different cell types: SP-positive/NFP-negative, SP-negative/NFP-positive, and SP/NFP-positive vestibular ganglion cells. Vestibular ganglion cells can thus be classified on the basis of size and immunocytochemical characteristics. These chemically distinct subpopulations of vestibular ganglion cells may be an indication of functional differences within vestibular afferent neurons.