Perilla (frutescens) seed oil, which is widely used as a source of vegetable oil in Korea, contains a strikingly large amount (58.4% of total fatty acids) of polyunsaturated linolenic acid (18 : 3) which is one of the essential fatty acids. Our hypothesis was that vitamin E contained in this oil would not be enough to prevent peroxidation of this polyunsaturated oil. A comparative study was carried out using rats and chicks devided into seven groups with various diet combinations emphasizing fat sources for the period of four weeks. The level of fat in each diet was 15% and animals were fed ad libitum. Various diet combinations were as follows; perilla seed oil and sesame seed oil with and without vitamin E supplementation, tallow as a saturated fat source and perilla seed hull group (10% at the expense of carbohydrate). The fat constituents of control group were consisted of 50% vegetable oil and 50% animal fat. A few important findings are as follows: 1. Rats fed perilla seed oil lost their hair focally around the neck and suffered from a bad skin lesion at the same place. In chicks, yellow pigmentation both of feather and of skin was clearly observed only in groups fed perilla seed oil with or without vitamin E supplementation. The basis of biochemical mechanisms of this phenomena remains as an important research interest. 2. The mean value for hematocrit was significantly lower for the chicks fed perilla seed oil than for those fed control diet. This result seems to be attributable to the effect on the red cell membrane known as peroxidation-hemolysis of vitamin E deficiency. 3. The serum cholesterol level was higher for the rats fed perilla seed oil than for those fed control diet, whereas in chicks the group fed perilla seed oil showed lower value than the control group indicating that different animal species could vary in their responses to the same diet. 4. In pathological examinations, the sign of hepatic fibrosis was seen in the perilla seed hull group and it was noticeable that the level of hepatic RNA was significantly increased in the rat recovering from vitamin E deficiency. It is hoped that more detailed studies on perilla seed oil and hulls will soon be carried out in many aspects especially i) at various levels of fat in the diet, ii) in relation to dietary selenium level and iii) to find an optimum level of dietary essential fatty acids in terms of P/S ratio using various animal species. In the mean time, the public should be informed to preserve this particular oil with care to minimize fatty acid oxidation and should be discouraged from overconsuming this oil. This study was supported by UB (United Board) Research Grant (Graduate School, Yonsei University, Seoul, Korea)