Aim To investigate the effects of aging on the molecular expression of α- and β₁-subunit of large-conductance Ca²⁺-activated K⁺ （BK_Ca） channels in rat mesenteric arterial smooth muscle cells. Methods Young （4～5 months）, middle-aged （15～16 months）, and old （22～24 months） male Wistar rats were used. In vivo blood pressure and in vitro endothelium-removed mesenteric arterial tension were measured. Immunohistochemistry and Western Blot assay were also conducted. Results The systolic blood pressure and pulse pressure were significantly increased with aging. The vascular tone caused by norepinephrine （NE） strongly increased in a concentration dependent manner in each group, and aging reduced the sensitivity to NE in mesenteric arteries （MA）. Selective BK_Ca channel blocker （Iberiotoxin） induced a marked increase of vascular tension in MA in all three age groups. However, these effects were greatly decreased in old animals. Western Blot showed that the protein expression of BK_Ca α- and β₁-subunit was significantly reduced with aging, and the suppression of β₁ subunits was larger than that of α subunits. Conclusions These data suggest that aging induces decrease of molecular expression of α- and β₁-subunit of BK_Ca channels in MA myocytes, in which the suppression of β₁-subunit is more pronounced than α-subunit. This unparallel downregulation of two subunits may be the mechanism underlying the aging-associated decrease of BK_Ca channel contribution in regulating MA basal tone.