The mouse brain auditory pathway has been dissected into five regions: geniculate bodies, posterior colliculi, superior olives, cochlear nuclei, and cochleas. The following analyses were made in these regions and in the auditory cortex: protein, glutamate, gamma-aminobutyrate, taurine, choline acetyltransferase, and glutamate decarboxylase. Taurine levels (nmol . mg of protein-) were highest in cortex (93) and geniculate bodies (60) and lowest in the cochlear nuclei (27) and cochleas (29). Concentrations of gamma-aminobutyrate (same units) were highest in the geniculate bodies (28), low in the superior olives and cochlear nuclei (9 to 10), and undetectable in the cochleas. The distribution of glutamate decarboxylase activity reflected that of gamma-aminobutyrate. The activities of choline acetyltransferase (nmol . of acetylcholine synthesized . h . -1 mg of protein-1) were highest in the superior olives (60) and low in the cochleas (3). These results are interpreted as biocyemical support for previous physiological and pharmacological identification of the olivo-cochlear bundle as cholinergic and the cochlear-nucleus neurones as non-cholinergic. The results also provide further evidence for a role of GABA in the posterior colliculi, but not in the cochleas.