Abstract: A method of treating a relatively aged animal to reverse age-related changes in the lipid composition of organ and tissue cells, such as heart muscle cells, and the ability of the animal to withstand respiratory stress. A suspension of small unilamellar vesicles composed predominantly of egg phosphatidyl choline is administered parenterally to the animal, preferably over a period of several days and at a dose level of between about 0.1 and 1 grams lipid per kg body weight per day. Changes in the heart muscle cells are reflected in decreased levels of serum creatine phosphokinase. Liposome administration is continued until the serum creatine phosphokinase level drops at least about 50%. Also disclosed are liposome treatment methods for increasing longevity and male fertility.