Abstract Potassium efflux through Ca 2+ -sensitive K + channels (K Ca channels) is increased in arterial smooth muscle cells from hypertensive rats, but the molecular mechanism is unknown. The goal of this study was to compare the levels of K Ca channel current between aortic smooth muscle cells from adult Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) and then use Western blot methods and ribonuclease protection assays to examine the expression and mRNA levels for the K Ca channel in these same vascular tissues. Whole-cell patch-clamp methods indicated a larger component of K Ca channel current, sensitive to block by iberiotoxin (100 nmol/L), in single aortic smooth muscle cells from SHR compared with WKY. Subsequent Western blot analysis using a site-specific antibody (anti–α 913–926 ) directed against the S9/S10 linker of the α-subunit of the K Ca channel revealed a 125-kD immunoreactive band in lanes loaded with either WKY or SHR aortic muscle membranes. The immunoreactive density of this band, which corresponded to the known molecular size of the α-subunit, was 2.2-fold greater in lanes loaded with aortic smooth muscle membranes from the hypertensive animals. However, despite this evidence for an increased expression and functional enhancement of K Ca channels in aortic smooth muscle membranes of SHR, ribonuclease protection assays with a 32 P-labeled riboprobe targeted against the S9/S10 linker of the K Ca channel α-subunit revealed no difference in mRNA levels for the α-subunit between WKY and SHR aortic tissue. These findings provide initial evidence that (1) an increased expression of K Ca channels may be a mechanism for the enhanced K Ca current in aortic smooth muscle membranes of SHR, and (2) the upregulation of K Ca channels in arterial muscle membranes during hypertension, which is regarded as a homeostatic mechanism for buffering vascular excitability, may rely on post-transcriptional events.