It has been proposed that Ca²⁺-activated K⁺ channels play an essential role in maintaining vascular tone during stretch of blood vessel. However, the underlying mechanism of stretch-induced change of Ca²⁺-activated K⁺ channel activities are still unknown. The present experiment was designed to investigate the effect of membrane stretch on these channels whose activity was measured from rabbit coronary smooth muscle cells using a patch clamp technique. Ca²⁺-activated K⁺ channel were identified by their Ca2+ and voltage dependencies and its large conductances as in other preparations. Perfusion of cells with a hypotonic solution, which mimics stretching the cell membrane by making a cell swelling, produced an increase in channel activity in cell-attached patch mode. The similar increase was observed when negative pressure was applied into the patch pipette for stretching the cell membrane within a patch area. In inside-out patch, stretch still increased channel activity even under the conditions which exclude the possible involvement of secondary messengers, or of transmembrane Ca2+ influx via stretch-activated cation channels. Pretreatment of arachidonic acid or albumin showed no effect on stretch-induced channel activation, excluding the possibility of fatty acids mediated channel activation during membrane stretch. These results indicate that the stretch may directly increase the activity of Ca²⁺-activated K⁺ channels in our experimental condition.