For understand-ing the regulation of sodium reabsorption, it is important to know whether the α₂- or α₃-isoform of Na-K-adenosinetriphosphatase (Na-K-ATPase) is expressed in mammalian kidney in addition to the predominant a-isozyme. Here we applied competitive polymerase chain reaction (PCR) for estimation of mRNAin parenchymal zones of rat kidney for comparison to high-affinity [³H]ouabain binding. The α₃-isoform mRNA was demonstrated to form 0.04-0.05% of the amount of arisoform mRNA in the cortex, medulla, and papilla of rat kidney. The α₂-mRNA was demonstrated in all three zones and constituted 0.03% of the amount of arinRNA in cortex. The abundance of the a. truncated mRNA was 0.1-0.8% of that of the ai-mRNA. The upper limit for expression of Na-K-ATPase isozyme with high ouabain affinity (dissociation constant, 69-141 nM) was 0.096-0.14% of the concentration of α₁β₁-Na-K-ATPase. Thus a small but well-defined pool of α₂- and α₃-isoforms constitutes £0.1% of the amount of oti-isoform at both the mRNA and protein level in rat kidney.