This study is the first to demonstrate that low concentrations of aqueous NO induce intracellular Ca2+ mobilization and an increase in secretory activity of rat pancreatic beta-cells. Application of NO solution (2 microM) resulted in a transient increase in the free intracellular Ca2+ concentration ([Ca2+]i) of isolated cells, as assessed by video ratio imaging and single wavelength microfluorimetry. Amperometry revealed a simultaneous increase in the release of preloaded 5-hydroxytryptamine from the isolated cells. The NO-induced Ca2+ response primarily involves mobilization of endoplasmic reticulum Ca2+ stores, since the response was retained when cells were transferred to low Ca2+ medium, and completely inhibited when cells were pretreated with 10 microM thapsigargin. The Ca2+ response was also inhibited when cells were incubated with a high concentration of ryanodine (200 microM), suggesting that Ca2+ mobilization is via a ryanodine-sensitive store.