The cellular distribution of the mRNA of the inwardly rectifying K+ channel KGA (GIRK1) was investigated in rat tissue by in situ hybridization. KGA was originally cloned from the heart and represents the first G protein-activated K+ channel identified. It is expressed in peripheral tissue solely in the atrium, but not in the ventricle, skeletal muscle, lung and kidney. In the central nervous system KGA is most prominently expressed in the Ammon's horn and dentate gyrus of the hippocampus, neocortical layers II-VI, cerebellar granular layer, olfactory bulb, anterior pituitary, thalamic nuclei and several distinct nuclei of the lower brainstem. The abundant expression of KGA in many CNS neurons supports its important role as a major target channel for G protein mediated receptor function.