1. Spontaneous miniature synaptic events were studied with tight-seal whole-cell recordings from CA3 neurons maintained in the hippocampal slice from immature rats (3-15 days). CA3 neurons suffer a constant, high-frequency barrage of inhibitory synaptic input. When inhibitory postsynaptic currents were suppressed by bicuculline, a smaller contribution from excitatory synapses was revealed. 2. Addition of tetrodotoxin (TTX) removed a persistent inward current and substantially reduced the baseline noise facilitating the detection of "miniature" excitatory currents. Addition of hyperosmotic media increased the frequency of spontaneous excitatory postsynaptic currents (EPSCs). 3. Under both physiological and elevated potassium conditions, individual spontaneous miniature EPSCs (10-30 pA amplitude) were composed of components mediated by N-methyl-D-aspartate (NMDA) and non-NMDA receptors as determined by their voltage dependence, time course, and sensitivity to selective antagonists. 6-Cyano-7-nitro-quinoxaline-2,3-dione (CNQX) or D-2-amino-5-phosphonovaleric acid (D-APV) shifted the amplitude distribution of miniature EPSCs to a smaller mode at both +40 mV and -40 mV. Similar to EPSCs recorded in CA1 neurons, the rise and decay times of the NMDA receptor component were slower than those of the non-NMDA component. The time course of the non-NMDA component was voltage independent. 4. In 13 of 21 neurons, no correlation existed between individual EPSC rise times and their corresponding halfwidth, peak amplitude, or decay time constant. This suggests that the large range of EPSC kinetics observed in each individual neuron was not due solely to cable attenuation of EPSCs widely distributed over the dendritic tree. Plots of the mean EPSC rise time against mean halfwidth for each cell, however, revealed a striking correlation, suggesting that in neonates, active synapses may be grouped in a restricted region of the dendritic tree and as such are subject to similar amounts of dendritic filtering. 5. The electrotonic length of CA3 neurons (L = 0.52) predicted that at this maturity the electrotonic compactness of the neuron facilitated voltage control over all but the most distal synapses. The reversal potential of the fast component of spontaneous events was close to 0 mV, whereas the reversal potential of exogenously applied kainate and NMDA was more positive. This discrepancy likely reflects a compromise of the voltage clamp by the activation of conductances distributed over the entire cell.(ABSTRACT TRUNCATED AT 400 WORDS)