Abnormally functioning K(+) channels during aging might contribute to increased arterial tone of vascular smooth muscle cells. However, changes in large conductance calcium-activated potassium (BKca) channel current density and channel protein composition during aging are not fully understood and are currently investigated here. Spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were grouped by age into very young (4-6 weeks), young (16-18 weeks), and adult (40-42 weeks) rats. BKca current was measured using a whole cell patch clamp in vascular smooth muscle cells from rat mesenteric arteries. BKca channel and subunits were measured using confocal imaging and real-time reverse transcriptase polymerase chain reaction techniques. Systolic and diastolic blood pressure increased significantly with age in both SHR and WKY rats. The expression of BKca α subunits increased in plasma membrane and cytoplasm in both SHR and WKY rats during aging, but had a larger increase in the cytoplasm in SHR than in WKY rats. BKca current density increased with aging in SHR but not in WKY rats. The relative abundance of BKca α and β1 mRNA transcripts changed minimally or not at all with aging in both SHR and WKY rats. In conclusion, plasma membrane BKca protein increased with aging in both SHR and WKY rats. Increases in cytoplasmic BKca protein and in BKca current were greater in SHR than WKY rats.