G protein-gated inwardly rectifying potassium (GIRK) channels are found in neurons, atrial myocytes and neuroendocrine cells. A characteristic feature is their activation by stimulation of Gi/o-coupled receptors. In central neurons, for example, they are activated by adenosine and GABA and, as such, they play an important role in neurotransmitter-mediated regulation of membrane excitability. The channels are tetrameric assemblies of Kir3.x subunits (Kir3.1-3.4 plus splice variants). In this study I have attempted to identify the channel subunits which contribute to the native GIRK current recorded from primary cultured rat hippocampal pyramidal neurons. Reverse transcriptase-polymerase chain reaction revealed the expression of mRNA for Kir3.1, 3.2A, 3.2C and 3.3 subunits and confocal immunofluorescence microscopy was used to investigate their expression patterns. Diffuse staining was observed on both cell somata and dendrites for Kir3.1 and Kir3.2A yet that for Kir3.2C was weaker and punctate. Whole-cell patch clamp recordings were used to record GIRK currents from hippocampal pyramidal neurons which were identified on the basis of inward rectification, dependence of reversal potential on external potassium concentration and sensitivity to tertiapin. The GIRK currents were enhanced by the stimulation of a number of Gi/o-coupled receptors and were inhibited by pertussis toxin. In order to ascertain which Kir3.x subunits were responsible for the native GIRK current I compared the properties with those of the cloned Kir3.1 + 3.2A and Kir3.1 + 3.2C channels heterologously expressed in HEK293 cells.