The localization of GABAA receptors was studied by immunohistochemistry in the nucleus tractus solitarii of the rat using a monoclonal antibody (bd17) against the beta-subunit. The pattern of distribution was compared with that of GABA-immunoreactive axons and nerve terminals. Positive staining for GABAA receptors was confined to regions near the surface of neuronal somata and their processes. The highest density of positive staining for GABAA receptors was seen in the central part of the rostral nucleus tractus solitarii where GABA-positive terminals were also rather dense. At both intermediate and caudal levels of the nucleus tractus solitarii, a moderate density of positive staining for GABAA receptors was located in the ventrolateral part, including the ventrolateral subnucleus. In these regions, the density of GABA-positive terminals was low. In the medial nucleus tractus solitarii, including the medial subnucleus, very little or no positive staining for GABAA receptors was detected, although many GABA-positive terminals were observed. The results suggest that the central part of the rostral nucleus tractus solitarii is controlled by the GABAergic system via GABAA receptors, but in the medial subnucleus of the nucleus tractus solitarii the GABA neurons appear to act via receptors that are not detectable by the antibody used.