Members of the protein Kinase C (PKC) family control a broad array of cellular functions by orchestrated phosphorylation of several different protein substrates. PKCα, a cytosolic member of the PKC family, upon exposure to phorbol esther (PMA) is translocated to the plasma membrane where it is brought into proximity of its substrates. Down-regulation of PKCα activity can occur through internalization by delivery of PKCα to the endosome compartment via a caveolae mediated process (1). We have shown previously that expression of the multidrug resistant transporter P-glycoprotein (Pgp) prevents the interaction of PKCα with caveolin-1, the major coat protein responsible for caveolae assembly, and consequently its internalization (2). We have now corroborated and extended these results. Down-regulation of PKCα-GFP is prevented when expressed in NIH-3T3-MDR-1 fibroblasts, a cell line permanently expressing Pgp. Transient expression of Pgp in NIH-3T3 and HEK293 cell lines, that do not endogenously express Pgp, confirms the finding and rules out any influence of the NIH-3T3-MDR-1 cell line selection process. Furthermore, by using mutations that abolishe Pgp transport activity, we show that Pgp function is not required for the retention of PKCα at the plasma membrane. Pgp is also a substrate of PKCα and it has been shown to be phosphorylated at eight sites(3). We show, by mutation of these sites, that PKCα-mediated phosphorylation of Pgp is not required for this phenomenon. Retention of PKCα at the plasma membrane might be due to either a direct molecular interaction with Pgp or mediated by a third molecular intermediary. Internalization of Na+/K+ATPase, a key plasma membrane transporter responsible for maintaining Na+ and K+ gradients, is dependent on PKCα phosphorylation (4). We show that retention of PKCα at the plasma membrane in a Pgp-dependent manner increases Na+/K+ATPase internalization. Since PKCα has a key role in cellular equilibrium and signal transduction, alteration of its trafficking is likely to greatly influence cell physiology as exemplified by our study on Na+/K+ATPase.