Based on measurements of GIRK channels expressed in Xenopus oocytes, it has been proposed that these are regulated by intracellular Na⁺. It is commonly assumed that GIRK channels in the heart are composed of Kir3.1 and Kir3.4 subunits in a 2:2 stoichiometry. On the other hand, evidence has been provided that homotetrameric wildtype Kir3.4 complexes form functional channels when overexpressed in atrial myocytes. Homomeric Kir3.4 channel currents can be identified by weaker inward rectification as compared to Kir3.1/Kir3.4 channel currents. In myocytes overexpressing Kir3.4 after adenoviral gene transfer, a background current (I_bir) developed within ~1 min upon getting into the whole cell mode. I_bir had the I/V characteristics of homomeric Kir3.4. I_bir was absent if the pipette solution was devoid of Na⁺ and saturated at 40 mM Na⁺. This is in line with previous studies showing regulation by [Na⁺]_i of GIRK channels expressed in oocytes. However, in myocytes neither endogenous background nor receptor-activated GIRK current are sensitive to [Na⁺]_i in a range between 0 and 40 mM. These data suggest that regulation by [Na⁺]_i does not play a role for endogenous GIRK channel currents in myocytes but reflects a property of homomeric Kir3.4 channels, challenging physiological relevance of the oocyte data.