The regulatory subunit of S. cerevisiae casein kinase II (CKII) is encoded of two genes, CKB1 and CKB2. Strains harboring deletions of either or both genes exhibit specific sensitivity to high concentrations of Na+ or Li+. Na+ tolerance in S. cerevisiae is mediated primarily by transcriptional induction of ENA1, which encodes the plasma membrane sodium pump, and by conversion of the potassium uptake system to a higher affinity form that discriminates more efficiently against Na+. To determine whether reduced ENA1 expression plays a role in the salt sensitivity of ckb mutants, we integrated an ENA1-lacZ reporter gene into isogenic wild-type, ckb1, ckb2, and ckb1 ckb2 strains and monitored beta-galactosidase activity at different salt concentrations. In all three mutants transcription from the ENA1 promoter remained salt-inducible, but both basal and salt-induced expression was depressed approximately 3- to 4-fold. The degree of reduction in ENA1 expression was comparable to that observed in an isogenic strain carrying a null mutation in protein phosphatase 2B (calcineurin), which is also required for salt tolerance. These results suggest that reduced expression ofENA1 contributes to the salt sensitivity of ckb strains. Consistent with this conclusion, overexpression of ENA1 from a heterologous promoter (GAL1) completely suppressed the salt sensitivity of ckb mutants. Induction of ENA1 expression by alkaline pH is also depressed in ckb mutants, but unlike calcineurin mutants, ckb strains are not growth inhibited by alkaline pH.