The A663T polymorphism of the alpha-subunit of the human epithelial sodium channel (hENaC) increases the functional and surface expression of alphabetagamma-hENaC in Xenopus laevis oocytes, and the context of this residue in the COOH terminus of alpha-hENaC is important for this effect. Query of a phosphoprotein database suggested that the alpha-T663 residue of hENaC might be a substrate for phosphorylation by casein kinase 1 (CK1). We tested the hypotheses that phosphorylation of alpha-T663-hENaC by CK1 would regulate the increased functional and surface expression of alpha-T663-hENaC vs. alpha-A663-hENaC in oocytes. General inhibition of CK1 with IC261 decreased the functional and surface expression of alpha-T663-hENaC, but not alpha-A663-hENaC. This decrease in alpha-T663-hENaC functional expression resulted from reduced delivery of alpha-T663-hENaC to the oocyte membrane. IC261 also inhibited the functional expression of alpha-T692-mENaC and a chimeric m(1-678)/h(650-669)alpha-T663, mbetagamma ENaC, but not alpha-A692-mENaC or m(1-678)/h(650-669)alpha-A663, mbetagamma ENaC. These data suggest that additional residues outside of the alpha-hENaC COOH terminus are important for modulation of alpha-T663-hENaC trafficking by CK1. Overexpression of CK1alpha did not alter functional expression of alpha-T663-hENaC. In contrast, modest overexpression of CK1delta enhanced, whereas higher levels of CK1delta overexpression inhibited alpha-T663-hENaC functional expression. CK1 did not phosphorylate the COOH terminus of either alpha-T663-hENaC or alpha-A663-hENaC in vitro. These data suggest that CK1, and perhaps specifically CK1delta, regulates the intracellular trafficking of the alpha-A663T functional polymorphism of hENaC indirectly by altering the rate of alpha-T663-hENaC biosynthesis and/or delivery to the plasma membrane.