Significant Ca²⁺ release was previously noted with the activation of L-type Ca²⁺ current in rat superior cerebral artery smooth muscle cells. Here we examined whether P2X current that is partly carried by Ca²⁺ also triggers Ca²⁺ release in this preparation. Application of P2X agonists evoked membrane currents and concomitant Ca²⁺ transients in whole-cell voltage-clamped single cells. The expected increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) was calculated from the time-integrated P2X current by assuming Ca²⁺ is the only charge carrier. The measured increase in [Ca²⁺]_i was plotted as a function of expected increase in [Ca²⁺]_i, and Ca²⁺ buffering power was obtained as a reciprocal of the linear fit to this relationship. Both ryanodine, a Ca²⁺-induced Ca²⁺ release inhibitor, and cyclic ADP ribose, a putative activator of Ca²⁺-induced Ca²⁺ release, had no significant effects on Ca²⁺ buffering power. These results suggest that Ca²⁺ influx through P2X receptors does not trigger significant Ca²⁺ release. Next we examined whether P2X responses influence the subsequent P2Y response. P2Y responses were characterized by measuring the rate of [Ca²⁺]_i increase obtained as the slope of the linear regression to the rising phase of the Ca²⁺ transient. During simultaneous application of P2X and P2Y agonist, the rate of [Ca²⁺]_i increase was facilitated or suppressed depending on the size of P2X receptor-mediated [Ca²⁺]_i increase. Membrane depolarization close to the Ca²⁺ equilibrium potential significantly promoted the rate of [Ca²⁺]_i increase. Our results suggest that the [Ca²⁺]_i increase and membrane depolarization caused by P2X current may regulate the subsequent P2Y response.