The amplitude of the whole-cell L-type Ca2+channel current recorded from vascular smooth muscle cells is reportedly greater in spontaneously hypertensive rats (SHR) than in Wistar-Kyoto rats (WKY). However, no study has examined properties of single Ca2+channels in arterial cells from these strains. To further test the hypothesis that activation of L-type Ca2+channels in arterial smooth muscle cells would be enhanced in SHR, we recorded single Ca2+channel currents in resistance mesenteric artery cells from SHR and WKY (8 to 9 weeks of age) using a cell-attached patch clamp technique. With 50 mmol/L Ba2+in the recording pipette, the depolarizing pulse from a holding potential of -40 mV evoked the single L-type Ca2+channel current. Opening of the single channels was more frequent in cells from SHR than from WKY. Single-channel conductance (20 pS) and open time (1 ms at 0 mV) did not differ in the two strains. The results suggest that an increased amplitude of the whole-cell current can be attributed to the enhanced opening of single Ca2+channels in the arterial smooth muscle cells from SHR compared with WKY. (Hypertension. 1998;31:1125-1129.)