Ceramide is a signalling lipid that has recently been implicated in the pathogenesis of obesity-related insulin resistance. Ceramide is synthesized by a family of six ceramide synthases in mammals (CerS1-6), each of which preferentially utilises fatty acids of particular acyl chain length (C14-C28). C16 ceramide has been shown to mediate insulin resistance in hepatic cells and adipocytes. Increased C18 ceramide, synthesized exclusively by CerS1, has been suggested to mediate insulin resistance in skeletal muscle, which is a primary site for glucose utilization and is significantly affected by high fat diet. In this regard, we have synthesized a specific inhibitor of CerS1, P053. P053 inhibits CerS1 with an IC50 of 0.5 µM in vitro and in cultured cells. To investigate the effect of P053 treatment on global lipid metabolism in vivo, 40 mice were fed with either normal chow diet or high fat diet and for each diet group, 10 mice were treated with 5mg/kg of P053. Semi-quantitative untargeted lipidomic profiling were performed on lipid extracts of muscle, liver, and brain tissue. In vivo, P053 treatment reduced C18 ceramide by 40% in muscle and liver with no effect on other sphingolipids, phospholipids, and diacylglycerol species. Interestingly, triacylglycerol levels were positively correlated with C18 ceramide levels and, were significantly reduced in muscle and liver tissues with P053 treatment. Our data suggest that C18 ceramide produced by CerS1 directly regulates triacylglycerol metabolism. In conclusion, we have created the first isoform-specific CerS inhibitor.