The present studies were designed to compare the effects of both dietary fat levels and P / S ratio on lipid components in plasma and tissues. Changes in plasma HDL-cholesterol, cholesterol and TG, and also in tissue cholesterol and TG were determined in young rats fed diets providing total dietary fat as 10%, 25% or 45% of calories and P / S ratio as 0.2 or 4.0. Plasma cholesterol levels were getting higher as dietary fat levels increased at P / S 0.2. Plasma cholesterol was lower in rats fed dietary fat either 25% or 45 %, each with P / S 4.0. But at 10% no change in plasma cholesterol were observed by P / S 4.0 because of a possible insufficiency of the absolute amount of PUFA. HDL-cholesterol was rather less sensitive to the modification of dietary fat level, but was reduced in rats fed diets of P / S 4.0 at either 25% or 45% fat, even though HDL-cholesterol were increased in the group of 10% with P / S 0.2. Total cholesterol per g- liver were significantly increased as dietary fat levels increased. Liver cholesterol levels were higher in rats fed diets of P / S 4.0 at higher fat levels (25% or 45%) which possibly suggested that a reduction of plasma cholesterol by high PUFA diet was not at least from a decreased synthesis of cholesterol in liver. However, in muscle no significant differences were found by feeding high P / S ratio at each levels of fat. At 10% fat level, compared to 25% or 45%, cholesterol level was lower in g-liver but higher in g- muscle. Plasma TG was decreased as more dietary fat were supplied at P / S ratio, but no consistant response obtained at low P / S ratio. TG per g-liver were reduced by feeding P / S 4.0 diet at 10% or 45% fat level but no differences were found in muscle. P / S 4.0 diet was more efficient in lowering plasma cholesterol TG and HDL-cholesterolt levels only if fat level was more than 25% of the total calories And young rats were more resistant to dietary fat modification.