In Escherichia coli heat stable enterotoxin (STa)-treated rat enterocytes, the rise of inositol triphosphate (IP3) preceded the rise of [Ca2+](i). Chelation of extracellular Ca2+ with EGTA and suspension of cells in Ca2+ free buffer both demonstrated the enterotoxin-induced initial rise of [Ca2+](i) with a concomitant loss of sustained phase. Furthermore, pretreatment of cells with dantrolene resulted in a decrease of the early response of [Ca2+](i), indicating the initial effect of the enterotoxin on the rise of [Ca2+](i) was mostly due to its mobilization from some IP3-sensitive intracellular stores.