Conversion of 1-14C linoleic acid and 1-14C γ-linole nic acid, and their esterification into phospholipids and triglycerides, were determined in adipose tissue in vitro by using epididymal fat pad from 4 groups of rats, normal, alloxan diabetic, CCl4 treated, and both aloxan and CCl4 treated rats. The results were discussed in special reference to conversion of these labeled fatty acids to arachidonic acid. The results were as follows. 1) The esterification of labeled fatty acids derived from 1-14C linoleic acid and 1-14C γ-linolenic acid to phospholipids and triglycerides showed decrease by more than 50% in the groups of rats treated with alloxan or both alloxan and CCl4 as compared with normal rats. However, no significant difference was found between normal group and CCl4 treated group. It was also found in those rats treated with alloxan or both alloxan and CCl4 that the decrease in esterification of fatty acids is stronger in triglycerides than in phospholipids. 2) When fat pad was incubated with 1-14C linoleic acid, the percentage of recovered cpm in 20:4 in phospholipids was found to be very low, showing 1.3-1.4% in the groups of normal, alloxan diabetic and both alloxan and CCl4 treated rats, and particular low value of 0.6% was found in the group of CCl4 treated rats. On the contrary, the percentage of recovered cpm in 20:3 showed increase in 2 groups of rats treated with alloxan and both alloxan and CCl4. And this results might suggest the disturbed desaturatuon of 20:3 in adipose tissue of these rats. While, the percentage of recovered cpm in 18:3 showed some decrease in 3 groups of rats treated with alloxan, CCl4, and both alloxan and CCl4. Conversely, the percentage distribution of 18:3-20:3 showed increase in the same 3 groups. On the other hand, in triglycerides fraction the percentage of recovered cpm in 18:2 occupied approximately 90% and percentage distribution of other fatty acids showed very low value. 3) When adipose tissue was incubated with 1-14C γ-linolenic acid the percentage of recovered cpm in 18:3 showed approximately 70% in phospholipid fraction of all groups of rats and the rest of the labeled fatty acids distributed in wide range. The percentage of recovered cpm in 20:4 was found to be 12% in normal group, 6.7% in alloxan treated group, 4.2% in CCl4 treated group, respectively. The decrease found in 3 pathologic groups was significant as compared to normal group. On the contrary, the recovered cpm in 20:3 showed increase in these 3 pathologic groups, particularly in 2 groups treated with alloxan and both alloxan and CCl4. It is considerable from this results that the desaturation of 20:3 is disturbed in this case too in alloxan groups. On the other hand, the percentage distribution of original 18:3 in triglycerides fraction occupied approximately 80%, so that the percentage of other labeled fatty acids were low in every 4 groups, moreover, no special difference was found in the percentage distribution of individual fatty acid among 4 groups of rats.