The Na-K-ATPase, or sodium pump, is comprised of two subunits, alpha and beta. Each subunit spans the lipid bilayer of the cell membrane. This review summarizes our efforts to determine how the two subunits interact to form the functional ion transporter. Our major approach has been to observe the potential for subunit assembly when one or both subunits are truncated or present as chimeras that retain only a limited region of the Na-K-ATPase. DNAs encoding these altered subunit forms of the avian Na-K-ATPase are expressed in mammalian cells. Monoclonal antibodies specific for the avian beta-subunit are then used to purify newly synthesized avian beta-subunits, and the presence of accompanying alpha-subunits indicates that subunit assembly has occurred. The ectodomain of the beta-subunit (approximately residues 62-304) is sufficient for assembly with the alpha-subunit, and a COOH-terminal truncation of the beta-subunit that lacks aminoacyl residues beyond 162 will assemble inefficiently. A maximum of 26 aminoacyl residues of the alpha-subunit are necessary for robust assembly with the beta-subunit, when this sequence replaces the COOH-terminal half of the loop between membrane spans 7 and 8 in the SERCA1 Ca-ATPase. This region of the Ca-ATPase faces the lumen of the endoplasmic reticulum. These findings encourage study of other related questions, including whether there is preferential assembly of certain subunit isoforms and how various P-type ATPases are targeted to their appropriate subcellular compartments.