1. Excitatory postsynaptic currents (EPSCs) were recorded in rat submandibular ganglion cells in vitro using the two-electrode voltage clamp technique. 2. The peak amplitude of EPSCs evoked by nerve impulses in single presynaptic fibres varied between 1.2 and 9.8 nA in different cells (mean = 4.6 +/- 2.6 nA; n = 23; -80 mV membrane potential; 22-25 degrees C). 3. Experiments were performed to re-investigate a previous hypothesis that different mechanisms underlie the generation of evoked versus spontaneous quantal EPSCs in submandibular cells. This hypothesis was based on the observation of different time courses of evoked and spontaneous EPSCs. 4. In agreement with previous studies, the time course of the decay phase of evoked EPSCs was described by the sum of two exponentials, with time constants tau 1 and tau 2 of 6.9 +/- 0.7 and 34.4 +/- 7.7 ms, respectively (n = 23; -80 mV membrane potential). 5. The double-exponential decay of evoked EPSCs persisted when transmitter release was reduced by bath addition of 100 microM cadmium chloride to the level of failures, one or several quanta. 6. Spontaneous EPSCs exhibited mean amplitudes of 81 +/- 24 pA (n = 5 cells; -80 mV membrane potential), and displayed an extremely wide range of peak amplitudes in the same cell (mean coefficient of variation (c.v.) = 0.37 +/- 0.09; n = 5 cells). In contrast to a previous report (see below), the decay phase of spontaneous EPSCs was found to exhibit a double-exponential time course with time constants similar to those of the evoked EPSC recorded in the same cell. 7. These results indicate that evoked and spontaneously released quanta of transmitter most probably act on the same population of postsynaptic receptors in submandibular ganglion cells. There is a large variability in the peak amplitudes of quantal EPSCs recorded in the same cell. This large variability is not due to electrotonic effects, since these cells lack dendrites.