The active Na+/K+ pump is composed of an alpha and a beta subunit. Until now, three putative isoforms of the beta subunit have been identified that share sequence similarity. We have expressed the beta 1 and beta 3 isoforms of Xenopus laevis Na+/K(+)-ATPase in Xenopus oocytes to compare functional properties of the Na+/K+ pump, including either of these two isoforms. Na+/K+ pump current, estimated as K(+)-induced outward current in voltage-clamped oocytes, was doubled by coexpression of alpha 1 subunits with either isoform of the beta subunit compared to expression of alpha 1 subunits alone. The kinetics of activation by external K+ and the voltage dependence of the electrogenic activity of the Na+/K+ pump were similar with both beta isoforms, indicating that both beta 1 and beta 3 isoforms can support expression at the oocyte surface of an active Na+/K+ pump with similar functional properties.