Trans polyunsaturated fatty acids are formed during processing of vegetable oils such as deodorization and frying. The oxidative metabolism of linoleic and alpha-linolenic acids and of their mono-trans isomers (9cis,12trans-18:2, 9trans,12cis-18:2 and 9cis, 12cis,15trans-18:3, 9trans,12cis,15cis-18:3, respectively) was studied in fasting rats. A single dose of 18.5 MBq of each [1-14C] labelled fatty acid (260 microg) was orally given to the animals. The 14CO2 expired was monitored during 24 h. Radioactive countings of the CO2-trapping agent were performed at regular intervals up to 24 h after oral administration of the radiolabelled fatty acid. Radioactive countings were also performed on several tissues (liver, heart, brain, kidneys, sus-epidydimal fat, gastrocnemian muscle, gastrointestinal tract and carcass). The 14CO2 production 24 h after oral administration of the fatty acid ranged from 55.5% to 68.7% of the radioactivity administered for the C18:2 isomers and from 69.7% to 73.5% for the C18:3 fatty acids. From 6 to 24 h, 14CO2 recovery was significantly higher after oral administration of 9cis, 12trans-18:2 than after giving both other octadecadienoic isomers. 14C retention per gram of tissue in the liver and in the heart was significantly lower after feeding 9cis,12trans-18:2 than after administration of both other C18:2 isomers. 14C retention per gram of tissue in the muscle was significantly lower after administration of both trans C18:2 isomers compared to linoleic acid. Neither 14CO2 recoveries nor 14C retentions were significantly different after administration of the three octadecatrienoic acids. The difference observed in 14CO2 recovery within the dienes was probably not due to a higher specificity of the enzymes involved in the beta-oxidation sequence for the Delta12trans double bond, as previously reported. Indeed, due to the labelling of the fatty acids on the carboxyl end, 14C values recorded in the CO2-trapping agent were only due to the first cycle of beta-oxidation.