The study was undertaken to determine the effects of heparin on hyperlipidemia induced by whole body Co-60 irradiation of 500 and 1,000 radson serum lipids and high density lipoprotein cholesterol in rabbits. The blood samples were collected once before and 12, 24, 48 and 72 hours after irradiation, 1,000 units of heparin were administered 11 and 23 hours after irradiation. Mild hyperlipidemia was observed 24 hours after irradiation in 500 rads irradiated group, however, the lipemia subsided in 48 hours. The lipid changes of serum in 500 rads irradiation with heparin treated animals were more milder than none-heparin treated animals. When 1,000 rads irradiation was applied, hyperlipemia appeared to increase from 12th hour to reach highest value in 24 hours after Co-60 irradiation. The curve of hyperlipemia in animals received 1,000 rads irradiation with heparin showed more milder rise and kept a shorter platear than non-heparin treated animals. The values of the serum high density lipoprotein cholesterol in 500 rads irradiated animals showed mild decrease since the 24th hour and returned to the normal range at 48 hours after irradiation, while no significant changes were observed in heparin treated animals. In 1,000 rads irradiated animals, decreasing tendency of high density lipoprotein cholesterol was seen 12 hours after irradiation and continued up to 72 hours after irradiation. A similar decreasing pattern of high density lipoprotein cholesterol was observed in 1,000 rads irradiation with heparin treated animals, but the depth of decrease was less than non-heparin treated animals.