The effect of reticuloendothelial hyperfunction on hypothermic preservation of lung was studied in dogs. In order to evaluate the viability after hemodynamic_ load in preserved isolated lung, observations were made on the rate of increase in weight, degree of edema,compliance and surface activity of lung. The results obtained as follows: l. In the group of activating of the reticuloendothelial system by injection of sodium thiosulfate intravenously before pneumonectomy and infusion of naphthionine through the pulmonary artery before hypothermic preservation of isolated lung the limit of preservation was eight hours whereas four hours in non-treated control group. 2.Therefore the method of activating of the reticuloendothelial system before and after pulmonary resection seems effective in preserving for prolonging the period of preservation of lung by means of inhibition of pulmonary edema. 3. Pulmonary surface activity is expected to be valuable as a method in evaluation of the viability of preserved lung along with compliance and rate of increase in weight of lung.[KTCS 1974;2:145-152]