An autoradiographic procedure has been used to determine the quantitative distributions of gamma-aminobutyric acid (GABAA and GABAB) receptor subtypes in rat brain. Although the concentrations of both receptor binding sites were similar in some brain regions GABAA sites generally outnumbered GABAB sites. The highest concentration of GABAA sites were detected in the frontal cortex, the granule cell layer of the cerebellum, the olfactory bulb and the thalamic medial geniculate. The highest concentration of GABAB sites occurred in the molecular layer of the cerebellum, the interpeduncular nucleus, frontal cortex, anterior olfactory nucleus and thalamic nuclei. In addition the globus pallidus, temporal cortex, lateral posterior thalamus, superior colliculus, pontine nucleus, raphe magnus, spinal trigeminal tract and substantia gelatinosa contained significantly more GABAB sites than GABAA sites. The physiological and pharmacological significance of this heterogeneity has yet to be determined.