Conjugated linoleic acid (CLA) and some trans fatty acids (FA) decrease tumor growth and alter tumor and host lipid uptake and storage. The goal of this study was to test the hypothesis that the acute inhibitory effects of CLA isomers and trans FAs on FA transport in tumors and white adipose tissue are mediated via an inhibitory G-protein coupled (GPC), FFA receptor (FFAR). Experiments were performed in hepatoma 7288CTC and inguinal fat pads in Buffalo rats during perfusion in situ. CLA isomers and trans FAs (0.03-0.4 mmol/L, in plasma) were added to the arterial blood, and FA uptake or release was measured by arterial minus venous difference. In hepatoma 7288CTC, the CLA isomers, t10,c12-CLA > (+/-)-9-HODE [13-(S)-hydroxyoctadecadienoic acid] > t9,t11-CLA, and the trans FAs, linolelaidic = vaccenic > elaidic, decreased cAMP content and inhibited FA uptake, 13(S)-HODE release, extracellular signal-regulated kinase p44/p42 phosphorylation, and [(3)H]thymidine incorporation. Other CLA isomers, c9,t11-CLA, 13-(S)-HODE, c9,c11-CLA, and c11,t13-CLA, had no effect. In inguinal fat pads, FA transport was inhibited by t10,c12-CLA = linolelaidic acid > trans vaccenic acid, whereas c9,t11-CLA had no effect. In both hepatoma 7288CTC and inguinal fat pad, addition of either pertussis toxin or 8-Br-cAMP to the arterial blood reversed the inhibitions of FA transport. These results support the idea that an inhibitory GPC FFAR reduces cAMP and controls FA transport by CLA isomers and trans FAs. Ligand activity is conferred by the presence of a trans double bond proximal to the carboxyl group.