Little is known about xenometabolites in human metabolism, particularly under exercising conditions. Previously. an exercise-modifiable, likely xenometabolite derivative, cis-3,4-methylene-heptanoylcarnitine, was reported in human plasma. Here, we identified trans-3.4-methylene-heptanoylcarnitine, and its cis-isomer, in plasma and skeletal muscle by liquid chromatography-mass spectrometry. We analyzed the regulation by exercise and the arterialto-venous differences of these cyclopropane ring-containing carnitine esters over the hepatosplanchnic bed and the exercising leg in plasma samples obtained in three separate studies from young. lean and healthy males. Compared with other medium-chain acylcarnitines, the plasma concentrations of the 3.4-methylene-heptanoylcarnitine isomers only marginally increased with exercise. Both isomers showed a more than twofold increase in the skeletal muscle tissue of the exercising leg: this may have been due to the net effect of fatty acid oxidation in the exercising muscle and uptake from blood. The latter idea is supported by a more than twofold increased net uptake in the exercising leg only. Both isomers showed a constant release from the hepatosplanchnic bed, with an increased release of the trans-isomer after exercise. The isomers differ in their plasma concentration, with a four times higher concentration of the cis-isomer regardless of the exercise state. This is the first approach studying kinetics and fluxes of xenolipid isomers from tissues under exercise conditions, supporting the hypothesis that hepatic metabolism of cyclopropane ring-containing fatty acids is one source of these acylcarnitines in plasma. The data also provide clear evidence for an exercise-dependent regulation of xenometabolites, opening perspectives for future studies about the physiological role of this largely unknown class of metabolites.