Dietary cis,trans-18 : 2 isomers impair desaturation and elongation of linoleic acid (Delta9cis,12cis-18 : 2), but little is known of their proportional partitioning between accumulation and oxidation. The present study was therefore designed to assess the accumulation and apparent oxidation of cis,trans-18 : 2 isomers compared with that of trans-18 : 1 isomers and Delta9cis,12cis-18 : 2 in rats. Accumulation is defined as whole-body increase in a fatty acid during a given period (i.e. final body content-initial body content). The apparent oxidation (disappearance) is defined as whole-body utilization of a fatty acid relative to its intake for a given period (intake-excretion-accumulation-longer-chain products)/intakex100). The animals were fed on a diet containing 15 % (w/w) partially hydrogenated rapeseed oil with 1.72 % energy as cis,trans-18 : 2 isomers and varying amounts of Delta9cis,12cis-18 : 2. The apparent oxidation of total cis,trans-18 : 2 isomers (72-76 % dietary intake) was greater than that of Delta9cis,12cis-18 : 2 (38-51 % dietary intake) but it was similar to that of total trans-18 : 1 isomers (78-82 % dietary intake). Among the four isomers, the apparent oxidation of Delta9trans,12trans-18 : 2 was greater than that of the other isomers including Delta9trans,12cis-18 : 2, Delta9cis,12trans-18 : 2 and Delta9cis,13trans-18 : 2. Accumulation of Delta5cis,8cis,11cis,15trans-20 : 4 and Delta5cis,8cis,11cis,14trans-20 : 4 derived from chain-elongation and desaturation of Delta9cis,13trans-18 : 2 and Delta9cis,12trans-18 : 2 was decreased when the dietary Delta9cis,12cis-18 : 2 supply was increased.