The complex EPSP-IPSP response of CA1 pyramidal cells to orthodromic activation of Schaffer collaterals-commissural afferent fibres was monitored during washout of Ca from hippocampal slices. The IPSP was clearly abolished before the EPSP and this occurred when the extracellular Ca([Ca2+]o) had fallen to between 1.03 and 0.7 mM. The loss of the IPSP was usually associated with a temporary increase in size of the EPSP and sometimes a membrane depolarization and the appearance of spontaneous activity. As [Ca2+]o fell further these effects were reversed and eventually the EPSP was abolished when [Ca2+]o had reached between 0.78 and 0.26 mM.