[3H]GABA quantitative autoradiography was used to examine the binding kinetics and regional distribution of GABAB receptors in rat brain. The regional distribution was compared to that of GABAA receptors. At 4 degrees C, [3H]GABA binding to GABAB receptors reached equilibrium within 45 min. The association and dissociation rate constants for GABAB binding to outer neocortical layers were 2.87 +/- 0.17 X 10(5) min-1 M-1 and 0.0966 +/- 0.0118 min-1, respectively, indicating a dissociation constant of 336 +/- 40 nM. Saturation binding studies in the same region yielded a dissociation constant for GABAB receptors of 341 +/- 41 nM while that of GABAA receptors was 92 +/- 10 nM. While the affinities of each type of GABA receptor were uniform across brain regions, the maximal number of binding sites for both types of GABA receptor varied across regions. The distributions of the two receptors in rat brain were different in the olfactory bulb, cerebellum, thalamus, neocortex, medial habenula and interpeduncular nucleus. Areas high in GABAB binding included the medial and lateral geniculates, the superior colliculus and certain amygdaloid nuclei. Binding to white matter tracts and ventricles was negligible. The distribution of GABAB receptors was in agreement with previously postulated sites of action of baclofen.