Glycine-immunoreactivity (Gly-I) was studied in the auditory brain stem of chick using a polyclonal antiserum to glycine conjugated to bovine serum albumin (Wenthold et al., 1987). During embryonic development, little Gly-I is present in nucleus magnocellularis (NM), nucleus laminaris (NL), or nucleus angularis (NA). In posthatch chicks, a few Gly-I terminals are found on nerve cell bodies in NM. Gly-labeled terminals are rare in NL and NA. When present in NL, they appear to be apposed to neuronal somata and not to dendrites. Occasionally, a Gly-labeled cell can be found in NM. After unilateral cochlea removal, Gly-labeled terminals are still present in NM. Thus, the cochlea does not appear to be the source of the glycinergic afferents to NM. The pattern of staining of Gly-I contrasts sharply with that of gamma-aminobutyric acid-immunoreactivity (GABA-I). The number of Gly-I terminals in NM, NL and NA appears to be much less than that of GABA-I. In addition, GABA-I terminals are very abundant around both the cell bodies and dendrites of neurons in NM and NL, whereas Gly-I terminals are found only on neuronal somata. Gly-I in NM also appears to be much less than that in its mammalian homologue, the anteroventral cochlear nucleus.