Endothelin (ET)-mediated Ca++ signaling in NG108-15 neuroblastoma x glioma cells was studied by measuring free intracellular Ca++ (Ca++i) levels with the fluorescent Ca++ indicator, fura-2. ET-1 produced biphasic increases in Ca++i consisting of a transient peak elevation followed by a sustained plateau phase. Both peak and plateau Ca++i responses to 5 nM ET-1 were reduced by depletion of extracellular Ca++. Peak responses were also attenuated by inhibitors of inositol phosphate metabolism, whereas plateau responses were affected by dihydropyridine Ca++ channel agonists and antagonists and by differentiation. These results suggest that peak Ca++i responses to ET-1 involve mobilization of Ca++ from inositol phosphate-sensitive intracellular stores and influx of extracellular Ca++ through nonclassical Ca++ channels, whereas plateau responses are mediated by Ca++ influx through dihydropyridine-sensitive, voltage-gated channels.