The differential distribution of glutamate (Glu), aspartate (Asp), glycine (Gly), gamma-aminobutyric acid (GABA) and taurine (Tau) was investigated in the cat's perihypoglossal nuclei. Serial semi-thin (0.5 micron) sections through the perihypoglossal nuclei were incubated with antisera raised against the mentioned amino acids with the aim of studying possible co-localization. In each experiment different measures were undertaken in order to screen for possible cross-reactivities, and all sections were processed together with test conjugates in order to ascertain the specificity of the antisera used. A very high proportion of the neurons in the perihypoglossal nuclei (about 90%) shows strong immunostaining for Asp and also displays distinct immunoreactivity for Glu in neighbouring sections. About 25% of the cells in the perihypoglossal nuclei are intensely immunostained for Gly, but very few cells show immunoreactivity for GABA. Only glial cells appear to be immunostained for Tau. Neurons that are Gly(+) also display Glu and Asp immunoreactivities. The neuropil of the perihypoglossal nuclei shows a high density of GABA(+), Gly(+) and Glu(+) puncta mainly representing stained axons and terminals. Fewer Asp(+) puncta and very few Tau(+) nerve terminal-like puncta are seen. Details of the regional distribution of immunopositive neurons and puncta within the perihypoglossal nuclei are described. The findings are discussed with particular reference to the possible role of the mentioned amino acids as transmitter substances in the known synaptic circuitry of the perihypoglossal nuclei.