The gunanine nucleotide-binding protein, Gi, which inhibits adenylyl cyclase, has recently been shown to have three subtypes of the α-subunit, termed Giα-1, Giα-2 and Giα-3. They share 87-94% amino-acid sequence homology1-10 and so are difficult to separate from one another. Among other functions11-14, purified preparations activate K+ channels15-21 but there is confusion over which of the subtypes activates the muscarinic K+ channels of the atrial muscle of the heart: Giα-3, also termed Gk15, has been shown to activate this channel but it is not clear whether Giα-122 does21 or does not23,24. To clarify this problem, we expressed the subtypes separately in Escherichia coli to eliminate contamination by other subtypes and tested the recombinant α- chains on atrial muscarinic K+ channels. Although we anticipated that only Giα-3 would have Gctivity, to our surprise all three recombinant subtypes were active, from which we deduce that the Gi subtypes are multifunctional.