This study was conducted using three groups of male rats to assess the effects of trans fatty acids on the desaturases of rat liver microsomes in vivo. The trans fatty acids fed rats showed significant decrease in the activities of delta 9 (p less than 0.001) and delta 6 (p less than 0.01) desaturase was normal as compared to the control group. The group of rats fed hydrogenated coconut oil showed only significant decrease in their liver microsomal delta 9 desaturase (p less than 0.05), whereas delta 5 and delta 6 desaturases were within the normal level of the control group. The level of trans fatty acids accumulated into the microsomal lipids of trans-fed rats reached to about 15.6% of their total fatty acids, of which 12.6% were represented as trans-18:1 acid. The trans-delta 9, delta 10, delta 11 and delta 12 18:1 isomers were the major isomers accumulated into the microsomes. The trans- delta 5, delta 6, delta 7, delta 15, and delta 16 18:1 isomers were detected only in small amounts ranging from 0.9 to 2.0%. This study shows that the dietary trans fatty acids are differentially incorporated into the liver microsomal lipids and act as inhibitors for delta 9 and delta 6 desaturases. The delta 6 desaturase is considered as the key enzyme in the conversion of the essential fatty acids to arachidonic acid and prostaglandins. This indicates that the presence of trans fatty acids in the diet may induce some effects on the EFA metabolism through their action on the desaturases.