Plasma membranes of sensory neurons from the olfactory and vomeronasal neuroepithelia of the male rat and olfactory neuroepithelium of the tiger salamander (Ambystoma tigrinum) have been examined, using the freeze-fracture technique, for the presence and morphology of orthogonal arrays of particles (OAP). Numerous OAP were scattered on the P-face of plasma membranes of the dendrites and cell bodies from rat vomeronasal sensory neurons. The OAP were 720 +/- 200 nm2 in area and they consisted of 4 to 20 particles whose centre-to-centre distance was about 7 nm. On the E-face, complementary orthogonal arrays of pits were observed. No OAP were detected in the olfactory sensory neurons of the rat. In the dendritic and perikaryal plasma membranes of the tiger salamander olfactory sensory neurons, OAP 2230 +/- 970 nm2 in area were observed on the P-face. The OAP consisted of 12 to 36 particles. The centre-to-centre distance of the particles was about 7 nm. In the olfactory receptor cell plasma membranes of this species, OAP formed complexes of 2 to 28 individual OAP, the longitudinal axes of which were usually arranged in parallel. Complementary complexes of orthogonal arrays of pits were observed on the E-face.