The development of gamma-aminobutyric acid-immunoreactivity (GABA-I) in nucleus magnocellularis (NM) and nucleus laminaris (NL) of the chick was studied by using an antiserum to GABA. In posthatch chicks, GABA-I is localized to small, round punctate structures in the neuropil and surrounding nerve cell bodies. Electron microscopic immunocytochemistry demonstrates that these puncta make synaptic contact with neuronal cell bodies in NM; thus, they are believed to be axon terminals. GABAergic terminals are distributed in a gradient of increasing density from the rostromedial to the caudolateral regions of NM. The distribution of GABA-I was studied during embryonic development. At embryonic days (E) 9-11, there is little GABA-I staining in either NM or NL. Around E12-14, a few fibers are immunopositive but no gradient is seen. More GABA-I structures are present at E14-15. They are reminiscent of axons with varicosities along their length, preterminal axonal thickenings and fiber plexuses. At E15, terminals become apparent circumscribing neuronal somata and are also discernible in the neuropil of both nuclei. In E16-17 embryos, terminals are the predominantly labeled GABA-I structures and they are uniformly distributed throughout NM. The density of GABAergic terminals increases in caudolateral regions of NM such that by E17-19, there is a gradient of increasing density of GABA-I terminals from the rostromedial to caudolateral regions of NM. The steepness of this gradient increases during development and is the greatest in posthatch (P) chicks. Cell bodies labeled with the GABA antiserum are located around the borders of both NM and NL and in the neuropil between these two nuclei. Occasionally, GABA-I neurons can be found within these auditory brainstem nuclei in both embryonic and posthatch chicks. Nucleus angularis (NA) contains some GABAergic cells. The appearance of GABA-I terminals around E15 is correlated in time with the formation of end-bulbs of Held on NM neurons. Thus, the ontogeny of presumed inhibitory inputs to chick auditory brainstem nuclei temporally correlates with, and could modulate the development of, excitatory auditory afferent structure and function.