We examined the effects of superoxide anion (O₂) on the intracellular Ca²⁺ concentration in cultured human nasal epithelial cells. The cells were exposed to O₂ by pretreatment with xanthine (X) and xanthine oxidase (XO); control cells were treated with X alone. When Ca²⁺-containing Krebs solution was reperfused in the thapsigargin-treated, store-depleted cells, reapplication-induced intracellular Ca²⁺ concentration elevation was significantly smaller in X/XO-treated cells than in the control cells, suggesting that O₂ impairs Ca²⁺ release-activated Ca²⁺ entry (CRAC). Bath application of ATP induced a steep Ca²⁺ transient in both control and X/XO-treated cells. However, the concentration-response curve of the ATP-induced Ca²⁺ transient was shifted to a higher concentration in X/XO-treated cells. The impairments of CRAC and ATP-induced Ca²⁺ transient induced by X/XO were reversed by superoxide dismutase. Furthermore, all these X/XO-induced effects were also observed in cells pretreated with pyrogallol, also an O₂ donor. These results indicate that O₂ impairs at least two mechanisms involved in Ca²⁺ mobilization in human nasal epithelial cells, i.e., CRAC and ATP-induced Ca²⁺ release.