Cytosolic calcium ([Ca2+](i)) oscillations are a nearly universal mode of signalling in excitable and non-excitable cells(1-4). Although Ca2+ is known to mediate a diverse array of cell functions, it is not known whether oscillations contribute to the efficiency or specificity of signalling or are merely an inevitable consequence of the feedback control of [Ca2+](i). We have developed a Ca2+ clamp technique to investigate the roles of oscillation amplitude and frequency in regulating gene expression driven by the proinflammatory transcription factors NF-AT, Oct/GAP and NF-kappa B. Here we report that oscillations reduce the effective Ca2+ threshold for activating transcription factors, thereby increasing signal detection at low levels of stimulation. In addition, specificity is encoded by the oscillation frequency : rapid oscillations stimulate all three transcription factors, whereas infrequent oscillations activate only NF-kappa B. The genes encoding the cytokines interleukin (IL)-2 and IL-8 are also frequency-sensitive in a way that reflects their degree of dependence on NF-AT versus NF-kappa B. Our results provide direct evidence that [Ca2+]i oscillations increase both the efficacy and the information content of Ca2+ signals that lead to gene expression and cell differentiation.