The hydrolytic activity of the ATPase bound to purified inner membrane vesicles of rat liver mitochondria can be increased threefold by washing extensively with a high ionic strength phosphate buffer. The specific ATPase activities of such phosphate-washed membranes are the highest reported to date for a mitochondrial membrane preparation (21-24 mumol of ATP hydrolyzed min-1 mg-1 in bicarbonate buffer at 37 degrees C). Deoxycholate (0.1 mg/mg of protein) extracts from these membranes a soluble, cold-stable ATPase complex which exhibits a specific activity under optimal assay conditions of 12 mumol of ATP hydrolyzed min-1 mg-1. This complex is not sedimented by centrifugation at 201000 g for 90 min, and readily passes through a 250-A Millipore filter. The ATPase activity of the soluble complex is inhibited 95% by 2.4 muM oligomycin. In addition, inhibitions of 60% or better are obtained in the presence of 1-8 muM dicyclohexylcarbodiimide, p-chloromercuribenzoate, venturicidin, and aurovertin. While a similar complex may be extracted with Triton X-100 this preparation is always lower in both specific activity and in inhibitor sensitivities than the complex extracted with deoxycholate. Detergents of the Tween and Brij series and other detergents of the Triton series are also much less effective than deoxycholate in solubilizing the oligomycin-sensitive. ATPase complex of rat liver. It is concluded that deoxycholate is superior to other detergents as an extractant of the oligomycin-sensitive ATPase complex of rat liver mitochondria, and that the complex extracted with deoxycholate possesses a closer similarity to the membrane-associated ATPase than does the complex extracted with Triton X-100. These studies document the first report of a detergent-solubilized, oligomycin-sensitive ATPase preparation from rat liver mitochondria.