Based on sequence homology with the rat atrial G protein-coupled muscarinic potassium channel (GIRK1 or KGA1/KGB1), a human cDNA encoding a G protein-activated inwardly rectifying K+ channel (HGIRK1) was isolated. The cDNA encodes a protein of 501 amino acids and shares 99% identity to rat GIRK1 in its total amino acid sequence. Southern blot analysis of genomic DNA indicates a high degree of conservation among various species. In the human population a useful NlaIII restriction fragment length polymorphism was found in the coding sequence of HGIRK1. Co-expression of HGIRK1 and the 5-HT1A receptor in Xenopus oocytes resulted in opening of the channel upon treatment with serotonin. HGIRK1 currents showed strong inward rectification and could be blocked by extracellular Ba2+. Northern blot analysis shows that HGIRK1 expression in human is most abundant in the brain, while lower levels are round in kidney and heart.