We have developed a system for measuring the dynamic changes in the cytoplasmic free calcium concentration [( Ca2+]i) in single vascular smooth muscle cells (VSMC) that is highly sensitive and does not cause cellular damage. Marked increases in [Ca2+]i in response to stimulation with caffeine and angiotensin II occurred in some VSMC of 4-week-old spontaneously hypertensive rats (SHR), in which the blood pressure and basal [Ca2+]i levels are not yet elevated. In 8-week-old rats, the basal [Ca2+]i level in VSMC was higher in SHR than in Wistar-Kyoto rats. Although the effects of high blood pressure on [Ca2+]i in vivo were expected to disappear during the passage culture, the [Ca2+]i in the fifth-passage cells was similar to that in the primary cells. These results suggest that the maintenance of high [Ca2+]i levels in VSMC of SHR is genetically regulated and is one of the mechanisms of hypertension in this strain, and that abnormal calcium regulation in VSMC of SHR is expressed even before overt hypertension.