The author studied the actions of ouabain and diphenylhydantoin sodium on the ATPase activity in mitochondrial fraction isolated from rabbit heart and compared with that of quinidine. The results obtained are as follows: 1) In studying the (Na++K+)-activated ATPase activity, the rabbit heart isolated was immediately frozen for 7-9 days (ageing of preparation) and thereafter the mitochondria1 fraction obtained by differential centrifugation technic was treated with solution A containing 0.15% deoxycholate for 24-48 hours at -10˚C before using in experiment. These methods increased the activity ratio to 0.87-0.98. 2) The (Na++K+)-activated ATPase activity in mitochondrial fraction of rabbit heart was not completely but markedly inhibited by ouabain. This inhibitory action of ouabain was moderately antagonised by K+ concentration at constant Na concentration. 3) Diphenylhydantoin sodium in concentration of 5 X 10-4 ~ 10-3M stimulated markedly not only Mg++-dependent ATPase activity but also (Na++K+)-activated ATPase activity and in concentration lower than 10-6M had little effect. However, this effect of diphenylhydantoin was markedly increased in the presence of Na+ alone rather than K+ alone, but lesser than that effect in the presence of both Na+ and K+, together. The stimulating effect of diphenylhydantoin was specifically antagonized by ouabaion. 4) When the rabbits were intravenously injected with ouabain and diphenylhydantion respectively, (Na++K+)-activated ATPase activity of rabbit heart of ouabain-treated group was much decreased and both (Na++K+)-activated ATPase and Mg++-activated ATPase activity were moderately increased in diphenylhydantoin-treated rabbit group. 5) The (Na++K+)-activated ATPase activity in mitochondrial fraction of rabbit heart was slightly inhibited by quinidine in high concentration of 10-4M, but nearly little effect was observed below the concentration of 5 X 10-5M. 6) It might be possible to conclude that diphenylhydantoin specifically antagonised the action of ouabain on the membrane ATPase, which is different from the action of quinidine.
