We have developed a series of inhibitors of glucosylceramide synthase that are Structurally based on the parent compound D-threo-1-phenyl-2-decanoylamino-3- morpholino-1-propanol (PDMP). These inhibitors provide useful tools for manipulating glycosphingolipid levels in cells and for elucidating questions associated with sphingolipid signaling. Recently, two highly active glucosylceramide synthase inhibitors, D-threo-3, 4 -ethylenedioxy-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol and D-threo-4-hydroxy-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol, were designed, synthesized, and studied. These inhibitors markedly reduced glycosphingolipid levels in MDCK cells without any accumulation of intracellular ceramide and associated growth inhibition. Subsequently, each inhibitor was evaluated for its ability to lower glycolipid levels in virally transformed lymphoblasts from a patient with a-galactosidase A deficiency. Both compounds significantly reduced neutral glycosphingolipid levels in the lymphoblasts without any morphological changes and growth inhibition. Furthermore, the inhibitors were applied to a mouse knockout model of Fabry disease. Inhibitor treatment blocked accumulation of globotriaosylceramide (Gb3) in the kidney, liver and heart of mice. In contrast to another glucosylceramide synthase inhibitor, N-butyldeoxynojirimycin, this treatment was not associated with any significant change in body weight or organ weight and without immunodepletion. These results suggest that these newest PDMP homologues are promising as therapeutic agents for the treatment of glycosphingolipid storage disorders.