We have investigated whether the initial rise in cytosolic free Ca2+, observed when cells are activated by certain receptor ligands, is mediated by inositol 1,4,5-trisphosphate (IP3). The chemotactic peptide formyl-methionyl-leucyl-phenylalanine increased cytosol Ca2+ (as measured with the Quin-2 technique) in human neutrophils, even in the absence of extracellular Ca2+, thus demonstrating Ca2+ mobilization. Similar findings were obtained with thyrotropin-releasing hormone in GH3 cells (pituitary cell line) and with carbamylcholine in RINm5F cells, an insulin-secreting line. IP3 levels were raised by the three agonists in their respective target cells within 5 s. The action of IP3 was assessed in digitonin-permeabilized cells. Such cell preparations display two steady-state levels, to which ambient free Ca2+ is lowered: one approximately 0.7 microM, maintained by mitochondria, another at 0.1-0.2 microM. The latter was ATP-dependent and was shown to be due to Ca2+ pumping by the endoplasmic reticulum. IP3 mobilized Ca2+ from the ATP-dependent pool, but not from mitochondria. The effect of IP3 is transient, which could be due to rapid degradation of the compound. However, when IP3 degradation by the permeabilized cells was measured, the half-time of degradation was short in RINm5F cells and GH3 cells, but much longer in neutrophils. IP3 degradation could thus not be the only explanation for the transient nature of the evoked Ca2+ mobilization. It is concluded that the early rise in cytosol Ca2+ following receptor-mediated cell activation is induced by IP3, thus attesting to the messenger function of this phospholipid breakdown product.