1. A combination of the techniques of microspectrofluorimetry and whole-cell patch clamp was used to investigate changes in cytoplasmic Ca2+ concentration (Cai2+) in single arterial smooth muscle cells on external application of ATP. 2. ATP applied to cells held under voltage clamp at --60 mV evoked an inward current and an associated rise in Cai2+. In the absence of extracellular Ca2+. ATP-activated inward currents were observed but there was no rise in Cai2+. 3. Pre-treatment of cells with noradrenaline or caffeine did not prevent the rise in Cai2+ on subsequent application of ATP. 4. The ATP-activated rise in Cai2+ was voltage dependent as outward currents evoked by ATP at positive membrane potentials were not associated with a change in Cai2+. 5. At --60 mV, the rise in Cai2+ due to ATP application was dependent on the magnitude of the ATP current response, such that Cai2+ increased by about 0.5 nM/pC charge transferred through ATP-gated channels. 6. The results suggest that ATP-gated channels in these cells admit sufficient Ca2+ in a physiological Ca2+ gradient to significantly elevate Cai2+. About 10% of the ATP-gated current may be carried by Ca2+ ions. Thus the ATP-activated channels have a dual excitatory function: depolarization due to Na+ entry promotes action potential discharge and voltage-gated Ca2+ entry, and also direct entry of Ca2+ through the ATP-activated channels.