Interaction of dietary Magnesium, Calcium and Polyunsaturated fatty acid(vegetable oils)on 3-Hydroxy-3-Methylglutaryl Co-A reductase activity was studied for a period of 30 days using isocalories and isonitrogenous as a basal diet . The subject rabbits were divided into 18 feeding groups. The results are summarized as follows: 1. The ratio of ${alpha}_1-lipoprotein$ par lipalbumin is 0.34 for control group, 0.38 the highest group fed 0.1 Mg(II) 10ml plus perilla oil and basal diet, the lowest 0.25 group fed 0.1M Mg(II) 5ml plus sesame oil and basal diet. 2. The ratio ratio of ${alpha}_2-lipoprotein$ per lipalbumin is 0.64 for control group. 0.95 as the highest for the group fed 0.1M Ca(II) 15ml plus sesame oil and basal diet, 0.1M Ca(II) 5ml plus perilla oil and basal diet. 3. The ratio of ${eta}-lipoprotein$ per lipalbumin is 0.71 for control group, the highest 0.81 for the groups fed 0.1M Mg(II) 10ml plus sesame oil and basal diet, the lowest 0.37 for the group fed 0.1M Mg(II) 15ml plus soybean oil and basal diet. 4. In serum triglyceride, control group was 129.5mg%, the highest 155.4mg% for the group fed 0.1M Ca(II) 5ml plus sesame oil and basal diet, the lowest 85.7mg% for the group fed 0.1M Mg(II) 10ml plus soybean oil and basal diet. 5. In serum cholesterol, control group was 96.7mg%, the highest 152.5mg% for the group fed 0.1M Ca(II) 10ml plus sesame oil and basal diet, the lowest 80.5mg% for the group fed 0.1M Mg(II) 15ml plus soybean oil and basal diet. 6. In case of HMG-CoA reductase activity, control group was 0.95, the highest 0.98 for the group fed 0.1M Ca(II) plus soybean oil and basal diet. 7. Interaction between metal(II) ions and polyunsaturated fatty acid(vegetable oil) are soybean oil>sesame oil>perilla oil, for Mg(II). soybean oil>perilla oil>sesame oil, for Ca(II). Therefore, it is invetigated that the interaction between metal ion and polyunsaturated fatty acid is the higher, the cholesterol level is the lower, and HMG-CoA reductase activity is increased.