Changes in the intracellular calcium concentration ([Ca²⁺]_i) are highly organized in both time and space. We examined the dynamic Ca²⁺ responses in smooth muscle cells of intact small mesenteric arteries of the rat using confocal microscopy. Intracellular Ca²⁺ oscillations were induced by superfusion with norepinephrine containing fluid, and the influence of membrane depolarization and hyperpolarization was examined. Stimulation with norepinephrine (3 × 10^­6 M) evoked asynchronous Ca²⁺ spikes (3.4 ± 0.2 spikes/min/cell, in 67 cells) which were usually accompanied by a rise in fluorescence. In the prolonged presence of norepinephrine, both spiking activity and fluorescence slightly but significantly decreased. Depolarization of the membrane potential, induced by increasing the K⁺ concentration in the superfusion fluid with 15 mM, synchronized the Ca²⁺ oscillations and significantly increased both spiking frequency and fluorescence. In contrast, hyperpolarization of the membrane potential with levcromakalim (3 × 10^­7 M) significantly reduced spiking frequency and fluorescence, to larger extent than observed in the presence of norepinephrine alone. Similar results were found with the vasodilatory neuropeptide calcitonin gene-related peptide (3 × 10^­9 M). Increasing the extracellular K⁺ concentration from 4 to 9 mM also significantly decreased the Ca²⁺ responses, suggesting a hyperpolarization of the membrane potential. Addition of the cannabinoid methanandamide caused a slight increase in spiking rate, accompanied with a slight decrease in fluorescence, suggesting little direct influence on the membrane potential. These findings show that stimulation with norepinephrine induces asynchronous Ca²⁺ oscillations in the smooth muscle cells of small mesenteric arteries. Our results further suggest that a depolarization is necessary to synchronize these responses, while hyperpolarizing the membrane potential tends to decrease the Ca²⁺ dynamics.