The purpose of the present study is to determine which descending projections to the cochlear nucleus may use gamma-aminobutyric acid (GABA) as a neurotransmitter. [3H]GABA (120 microM) was injected into the cochlear nucleus of albino and pigmented guinea pigs. After survival times between 0.25 and 16 h, the brain stems were prepared for light microscopic autoradiography. After 2 h survival there was a pulse of label, which progressed through the fibres from the cochlear nucleus to the ipsilateral superior olive. After 5 h, retrogradely labelled neuronal cell bodies and fibres were located in the superior olivary complex bilaterally. In the trapezoid body, clusters of labelled cells were seen in the lateral nucleus, ipsilaterally, and in the ventral nucleus, bilaterally. Also there were labelled cells in the ipsilateral dorsal and anterolateral periolivary nucleus. Large and small cells of several types were labelled. Survival times of 10 h or more resulted in very light, diffuse labelling. Projections to the cochlear nucleus labelled by retrograde transport of horseradish peroxidase that did not take up [3H]GABA included the inferior colliculus, bilaterally, and the cochlear nucleus and periolivary nuclei (other than ventral trapezoid nucleus), contralaterally. The selective labelling of cell groups in the superior olive with the moderately low concentration of [3H]GABA used is consistent with the high-affinity uptake of [3H]GABA by synaptic endings in the cochlear nucleus and its retrograde by transport GABA-ergic neurons. This provides evidence for a descending projection system for inhibitory feedback from the superior olive to the cochlear nucleus.