1. The effects of gamma-aminobutyric acid (GABA), microionophoretically applied onto neurons within the dorsal and posteroventral divisions (i.e., caudal regions) of the cochlear nucleus (CN), were studied during postnatal development in kittens with the use of extracellular recording techniques. Approximately 80% of all neurons encountered within the caudal CN responded to exogenously applied GABA regardless of neuronal response type or postnatal age. 2. GABA reduced acoustically evoked as well as spontaneous discharge rates in a dose-dependent manner at all ages studied, and generally abolished discharge activity at sufficiently high doses (i.e., ejection currents). Dose-response curves generated during acoustic stimulation by varying GABA ejection current were sigmoidal at all ages studied, and the range of slopes relating discharge rate to applied currents increased during the first 10 postnatal days. 3. Neural thresholds to acoustic stimuli were elevated, and slopes of discharge-rate-versus-sound-pressure-level curves were depressed regardless of age when GABA was microionophoresed onto CN neurons. 4. GABA's capacity to reduce spontaneous or acoustically evoked discharge rates was a voltage-dependent phenomenon directly related to control discharge rates (i.e., efficacy was high when discharge rates were high) for neurons recorded from both immature and mature animals. 5. A small set of neurons recorded from animals younger than 2 wk exhibited prolonged GABA "activation" and "deactivation" times (i.e., times required to achieve 90% of the maximal effect evoked by GABA and to recover from that effect, respectively) and may represent a group of actively differentiating units. 6. Bicuculline microionophoresis effectively blocked the actions of exogenously applied GABA and endogenous GABA, which, presumably, was synaptically released as a result of acoustic stimulation. The actions of bicuculline were dose dependent in animals ranging in age from 2 postnatal days to adulthood. These results suggest that recognition sites for GABA and bicuculline and the ionophore associated with the GABA receptor are present and functionally coupled in the caudal CN before the developmental period during which the full complement of inhibitory projections form synaptic contacts with these cells.