Trans-fatty acids exacerbate essential fatty acid deficiency in experimental animals by interfering with the metabolism of linoleic (18:2n-6) and alpha-linolenic (18:3n-3) acids and, hence, the eicosanoid production. The interfering effect depends on the type of trans-fatty acid. The detrimental effect of trans-fatty acid, however, apparently disappears when dietary linoleic acid is sufficiently supplemented. Trans-fatty acids appear to disturb fetal growth and birth weight, possibly by the same mechanism.