1. The cause of the increased frequency of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) resulting from anoxia was investigated in CA1 neurons of the in vitro rat hippocampal slice. These neurons were examined by whole-cell patch-clamp recording, and hypoxia was induced by switching the perfusion of the slice from oxygenated artificial cerebral spinal fluid (ACSF) to ACSF saturated with 95% N2-5% O2. Except where noted, experiments were carried out in ACSF containing 1 microM tetrodotoxin (TTX). 2. Although anoxia resulted in a significant increase in the frequency of mEPSCs, the amplitude, rise time, and half-decay time of the mEPSCs were unchanged. This increase in frequency indicates that there is a change in presynaptic neurotransmitter release mechanisms, probably an increase in calcium concentration, soon after the onset of anoxia. The unchanged kinetics and amplitude of the mEPSCs indicate that anoxic-induced synaptic changes are not a result of changes in the postsynaptic glutamate receptor. 3. When hippocampal slices were exposed to anoxic conditions in ACSF with calcium excluded, an increase in mEPSC frequency equal to that in normal ACSF was observed. When 0.2 mM of CdCl2 was added to the zero-calcium ACSF, anoxia still resulted in increases in mEPSC frequency equal to those of normal ACSF. It is therefore concluded that the anoxia-induced increase in mEPSC frequency does not result from an increase in a transmembrane calcium influx. The zero-calcium plus 0.2 mM CdCl2 ACSF solution completely abolished orthodromically elicited synaptic potential (in the absence of TTX), indicating that calcium currents that mediate normal orthodromic transmitter release were completely abolished in the latter experiments.(ABSTRACT TRUNCATED AT 250 WORDS)