GIRK1/KGA (referred to as GIRK1) is a member of the inwardly rectifying K+ channel family and is activated by G protein-linked receptors. The activation of this channel leads to hyperpolarization of the plasma membrane and is fundamental to the control of atrial and neuronal excitability. RNA blotting studies have shown that GIRK1 is expressed in the atrium of the heart and in the brain. We have used in situ hybridization histochemistry to characterize the pattern of expression of GIRK1 mRNA in adult rat heart and brain. In heart, expression of GIRK1 mRNA is homogeneous throughout the atria. There is no significant ventricular expression, although the conduction bundles were not specifically identified. GIRK1 mRNA expression in the brain is widespread with highest levels in the cortex, septum, hippocampus, thalamus, amygdala, cerebellum, and many nuclei of the midbrain and hindbrain, including red nucleus, inferior colliculus, pontine nucleus, nucleus of the solitary tract, and multiple cranial nerve nuclei (motor and sensory components). This detailed map of GIRK1 mRNA expression provides a basis for further study of this important new family of K+ channels.