An inhibitor of glucosylceramide (GlcCer) synthase, l-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), has been reported to deplete cells and mice of their glucosphingolipids. This inhibitor has proved useful for the elucidation of the many functions of this lipid family [reviewed by Radin, N.S. & Inokuchi, J. (1991) Trends Glycosci Glycotechnol 3, 200–213]. In the present study, we have synthesized homologs of PDMP having different acyl chains (C₆-C₁₈) and compared their effectiveness for the inhibition of GlcCer synthase in vitro and their inhibition of GlcCer, protein, and DNA synthesis in cultured MDCK (Madin-Darby canine kidney) cells. Using MDCK homogenates and mouse brain and liver microsomes, we found that the C₆ compound was relatively inactive and that the longer chain compounds did not differ much in inhibitory power. However, the use of intact MDCK cells showed that the longer chain homologs were much more effective in inhibiting GlcCer synthesis, cell growth, and incorporation of [³H]thymidine. Tests with two radioactive homologs showed that the inhibitor with a longer acyl chain was taken up much more effectively by MDCK cells and that this difference explains the much greater effectiveness of this homolog in intact cells. The inhibitors were effective when solubilized either with a nonionic detergent or with bovine serum albumin. The extent of decrease in DNA synthesis was not directly proportional to the decrease in cellular glucosylceramide, possibly because only a low level of the glycolipid is needed for DNA synthesis.