Immunocytochemical procedures employing the unlabeled antibody enzyme (PAP) method were used to visualize those neuronal elements in the basilar pontine nuclei (BPN) and nucleus reticularis tegmenti pontis (NRTP) of the rat which contain glutamic acid decarboxylase (GAD), a key enzyme involved in the synthesis of the neurotransmitter gamma-aminobutyric acid (GABA). Neuronal somata, axons, and axon terminals in the BPN and NRTP exhibited GAD-immunoreactivity. Both thick and thin varieties of labeled axons and terminals were distributed in varying densities throughout the BPN and NRTP. The greatest accumulation of labeled terminals was noted in the ventrolateral and lateral border regions of the BPN while a slightly less dense aggregation was observed along the ventral, ventromedial and midline regions of the pontine gray. Labeled fibers and terminals were also observed in the dorsal and ventral peduncular regions as well as central portions of the lateral, ventral, and medial pontine areas. Axonal and terminal labeling was present throughout the NRTP but no focal increases in density similar to those in the BPN were apparent. No obvious GAD-labeled fiber bundles could be observed to enter the BPN or NRTP. However, small fascicles of labeled axons were seen to course ventrally around the dorsolateral aspect of the cerebral peduncle to reach lateral pontine areas while other labeled axons descended through clefts in the mid-portion of the cerebral peduncle or passed through the medial lemniscus and around the medial portion of the cerebral peduncle to enter the pontine gray.