– Clofibric acid-d4 is intended for use as an internal standard for the quantification of clofibric acid by GC- or LC-MS. Clofibric acid is a peroxisome proliferator-activated receptor α (PPARα) agonist (EC50 = 50 µM in a transactivation assay) and the active metabolite of clofibrate. It is formed from clofibrate by tissue and serum esterases. Dietary administration of clofibric acid (0.067-0.22%) reduces serum cholesterol, phospholipid, and triglyceride levels in rats. It decreases glutamate oxaloacetate transaminase (GOT) levels and increases glutamate pyruvate transaminase (GPT) and lactate dehydrogenase (LDH) levels, markers of xenobiotic stress, in the plasma of carp (C. carpio) when administered in tank water at a concentration of 10 µg/L. Clofibric acid has been found in wastewater effluent.