Modulation of intracellular Ca2+ concentration [( Ca2+]i) is a signal for the contraction of vascular smooth muscle cells responding to vasoreactive substances. We prepared confluently cultured smooth muscle cells from rat aorta, loaded them with Ca2+ sensitive fluorescent dye, fura-2, and measured the [Ca2+]i transient by microscopic spectrofluorometry. The [Ca2+]i was distributed heterogeneously in cytosol. Angiotensin II (10 nM) transiently doubled the [Ca2+]i. It was also increased by arginine-vasopressin (10 nM), even after stimulation by angiotensin II was saturated. In contrast, acetylcholine (10 microM) or rat atrionatriuretic peptide (10 nM) did not change the [Ca2+]i in the same detecting field of the same cell, contradicting previous reports.