Ins(2,4,5)P3 is a relatively stable analog of the intracellular messenger Ins(1,4,5)P3, which has proven useful in those analyses of InsP3 receptor behaviour that require either prolonged exposure to agonist or that seek to prevent formation of potentially active metabolites, notably Ins(1,3,4,5)P4. There is, however, some evidence suggesting that Ins(1,4,5)P3 and Ins(2,4,5)P3 may have different effects on Ins PS receptors. In Xenopus oocytes, for example, the two isomers evoke different patterns of Ca2+ spiking that do not appear to result from their different susceptibilities to metabolism (Berridge & Potter, 1990). We have used rapid superfusion of permeabilized rat hepatocytes loaded with 45Ca2+ and then immobilized on a filter sandwich (Marchant & Taylor, 1996) to examine the kinetics of InsPs-stimula ted 45Ca2+ efflux with high temporal resolution. Both Ins(1,4,5)P3 and Ins(2,4,5)P3 caused concentration-dependent accelerations to a peak rate of 45Ca2+ release that was abruptly followed by bi-exponential decays in the release rates. For both isomers, the extent of5Ca2+ release (EC50=477±21 nM, n=4 and 12.6μM, for Ins(1,4,5)P3 and Ins(2,4,5)P3 respectively) and the peak rate of release (EC50=941±22 nM, n=3 and 23.1μM) differed by only 2-fold in their sensitivity to InsP3. Both isomers therefore cause quantal Ca2+ mobilization. The peak rate of 45Ca2+ mobilization evoked by a maximal concentration (300μM) of Ins(2,4,5)P3 was only 65.0±2.7% (n=3) of that evoked by a maximal concentration (10μM) of Ins(1,4,5)P3. Furthermore, a maximal concentration of Ins(2,4,5)P3 (333 μM) reduced the peak rate of 45Ca2+ efflux evoked by 10 μM Ins(1,4,5)P3 to 79.9±5.0% (n=3) of its control value. We conclude that Ins(2,4,5)P3 is partial agonist of liver Ins PS receptors with -65% of the intrinsic activity of Ins(1,4,5)P3.