The effects of arginine vasopressin (AVP, 10<sup>-7</sup> M) on the spatial dynamics of intracellular [Ca<sup>2+</sup>] in single cultured smooth muscle cells of the rat aorta were studied by digital imaging microscopy using the fluorescent Ca<sup>2+</sup> indicator fura-2. The nuclear and cytosolic regions were distinguished by the fluorescent image excited at 380 nm. Changes in intracellular [Ca<sup>2+</sup>] were expressed as percent increases in the ratios of fluorescence intensity at 500 nm excited by 340 and 380 nm. AVP increased the nuclear and cytosolic [Ca<sup>2+</sup>] in Ca<sup>2+</sup>-containing (control) (285 ± 27 and 172 ± 22%, respectively) or Ca<sup>2+</sup>-free (203 ± 26 and 121 ± 15%, respectively) solutions. However, caffeine (20 m M) and ryanodine (20 µ M)greatly attenuated the [Ca<sup>2+</sup>] increase induced by AVP in both regions (61 ± 21 and 42 ± 15%, respectively). On the ratio image, the nuclear region was discriminated from other regions at the peak response to AVP in preparations treated with caffeine and ryanodine, whereas the outline of the nuclear region was indistinct in untreated preparations. The finding implies that caffeine- and ryanodine-responsive Ca<sup>2+</sup> storage sites may exist in the region surrounding the nucleus. The results suggest that the region surrounding the nucleus may be one of the important Ca<sup>2+</sup> storage sites with regard to the responses of rat aortic smooth muscle cells to AVP.