Madin-Darby canine kidney (MDCK) cells were mutagenized and variants resistant to 10, 160, and 2000 times the ouabain lethal dose for wild type cells selected. The phenotypes were stable in the absence of selection. The frequencies with which variants were recovered were consistent with genetic alterations being responsible for drug resistance. It was shown that 50% of the (Na+, K+)-ATPase activity present in mutant cells had a higher Kd for ouabain than normal while 50% remained wild type for ouabain binding. Wild type MDCK cells were measured to have 2 X 10(6) ouabain binding sites per cell with a Kd for the drug of 0.6-1.0 X 10(-7) M. The novel (Na+, K+)-ATPase activities in the mutants demonstrated Kd values for ouabain of 10(-5) M, 3 X 10(-4) M, or 3 X 10(-3) M for the different mutant classes tested. The rate of synthesis of the (Na+, K+)-ATPase as well as the total amount of enzyme per unit of cell protein was unaltered in the mutants. Comparison of the alpha subunit of the enzyme, known to contain the ouabain-binding site, by sodium dodecyl sulfate-gel electrophoresis did not reveal any difference in the size of this subunit in mutant versus wild type cells.