The transduction of many hormonal and sensory stimuli is mediated by an increase in the intracellular concentration of inositol 1,4,5-trisphosphate ([IP3]). The interplay of [IP3], the intracellular free Ca2+ concentration, and the IP3 receptor/channel is thought to lead to the Ca2+ spikes and waves observed in many cell types. Despite the importance of IP3 in Ca2+ signaling, [IP3] in a single cell before, during, and following application of PLC-activating stimuli is not known. No method exists to measure [IP3] in single cells. Most estimates of [IP3] are based on homogenates of large numbers of cells, and represent the average [IP3] of an asynchronous population. This may in part explain the wide range of predicted, physiologic [IP3]. Measuring [IP3] in single cells is fundamental to understanding Ca2+ signaling. We combined capillary electrophoresis with a permeabilized IP3-detector cell to develop a method with the sensitivity to measure [IP3] in small regions of a single Xenopus oocyte. We will present early results obtained with this new technique.