Abstract The endogenous ATPase activity of rat brain mitochondria was stimulated 30-50 per cent by 15-50 mm concentrations of NaCl or Na acetate. The Na stimulation was completely abolished by small amounts of oligomycin but unaffected by ouabain. The differential effects of these inhibitors indicated that the Na-induced ATPase activity did not result from microsomal or synaptosomal contamination of mitochrondria. Sodium salts decreased the stimulatory effects of DNP, gramicidin, or Ca, but not that of Mg on the endogenous ATPase activity. These interactions were specific for Na+ as the corresponding salts of K+ did not affect the endogenous ATPase or inhibit the DNP-stimulated ATPase activity except at high concentrations. The Na-induced increases in ATPase activity and respiration were more sensitive to aging of the mitochondria than were ADP/O and respiratory control ratios, or the DNP-induced ATPase activity. These results suggest that Na+ may interact in brain mitochondria with the same high-energy intermediate of oxidative phosphorylation proposed for DNP and Ca.