The oligomeric state of canine renal Na +/K +-ATPase solubilized by octaethylene glycol n-dodecyl ether (C 12E 8) was studied by means of low-angle laser light scattering photometry coupled with high-performance gel chromatography (HPGC). At around 0°C the solubilized enzyme was separated into the (αβ) 2-diprotomeric and αβ-protomeric protein components with M r values of 302 000 ± 10 000 and 156 000 ± 4000, respectively, in approximately equal quantities. As the temperature of chromatography was increased toward 20°C, the two protein components converged into a single major component. The M r of this component depended on the monovalent cation included in the elution buffer, and was 255 000 or 300 000 in the presence of 0.1 M NaCl or 0.1 M KCl, respectively. A computer simulation technique showed that the solubilized enzyme was in a dissociation-association equilibrium of 2 promoters ⇌ diprotomer at 20°C, and the difference in apparent M r of the solubilized enzyme between the two species of monovalent cation was interpreted by an association constant ( K a) in the presence of 0.1 M KCl that was about 50-fold larger than in the presence of 0.1 M NaCl. In order to measure ATPase activity and M r of the solubilized enzyme simultaneously, a TSKgel G3000SW column had been equilibrated and was eluted with an elution buffer containing 0.30 mg/ml C 12E 8 and 60 μg/ml phosphatidylserine (bovine brain) as well as the ligands necessary for the enzyme to exhibit the activity at pH 7.0 and 20°C. The solubilized enzyme was always eluted as a single protein component irrespective of the amount of the protein applied to the column, ranging between 240 and 10 μg. The M r of the protein component, however, decreased from 214 000 to 158 000 with the decrease of the protein amount. The specific ATPase activity, however, remained constant at a level of 64 ± 4% of that of the membrane-bound enzyme even in the range of protein concentration sufficiently low as to allow the enzyme to exist only in the protomeric form. Thus, the αβ-protomer is concluded to be the minimum functional unit for the ATPase activity. The value of K a obtained from the concentration-dependent dissociation curve was 5 · 10 5 M −1 for the enzyme turning over, and 1.1 · 10 7 M −1 for the enzyme inhibited with ouabain. It was discussed, based on the values of K a obtained, that the enzyme would exist as the diprotomer or the higher oligomer in the membrane.