The effects of dietary cis and trans alpha-linolenic acid (18:3n-3) on the FA composition of plasma, red blood cell, and liver phospholipids were studied in newborn piglets. Animals were fed for 14 d with one of three diets: a control diet (group A) containing cis 18:3n-3 at a level of 2.0% of total FA, a diet (group B) in which a part of the 18:3n-3 acid was isomerized (1.3% of cis 18:3n-3 and 0.7% of trans 18:3n-3), or a diet (group C) with 2.0% cis 18:3n-3 and 0.7% trans 18:3n-3. Feeding animals with diets containing trans 18:3n-3 resulted in the presence of trans isomers of 18:3n-3, trans isomers of EPA, and trans isomers of DHA in phospholipids; however, the level of total trans n-3 PUFA in tissues was less than 0.3% of total tissue FA. In group B, the reduction of dietary amounts of cis 18:3n-3 was associated with a decrease in individual and total cis n-3 PUFA. In contrast, in group C there was no decrease in tissue n-3 PUFA despite the increased dietary level of trans 18:3n-3. These results suggest that the isomerization of a part of dietary n-3 PUFA, leading to the reduction of their levels in the diet, could induce a decrease in n-3 PUFA in phospholipids. The physiological effects of trans PUFA are not known and should be considered in future studies.