1. Excitatory postsynaptic currents (EPSCs) in parasympathetic preganglionic neurons (PGNs) were examined by the use of the whole cell patch-clamp recording technique in slice preparations of the neonatal rat lumbosacral spinal cord. Synaptic responses were evoked in PGNs by extracellular stimulation of a neighboring interneuron. 2. Stimulation of interneurons medial to the sacral parasympathetic nucleus (SPN) elicited EPSCs or inhibitory postsynaptic currents in 58 and 11%, respectively, of PGNs. Stimulation of interneurons dorsal to the SPN evoked EPSCs in 70% of PGNs. 3. EPSCs occurred at short latency (2.1 ms) and were usually elicited in an all-or-none manner, indicating that they were monosynaptic and mediated by a single interneuron (i.e., unitary). 4. EPSCs were mediated by both non-N-methyl-D-aspartate (non-NMDA) and NMDA receptors. 5. Unitary excitatory postsynaptic potentials evoked by single stimuli did not induce action potentials in PGNs, but repetitive stimulation (> 20 Hz) of the single interneurons could evoke firing of PGNs. 2-Amino-5-phosphonovalerate, an NMDA receptor antagonist, reduced the synaptic depolarization induced in PGNs by high-frequency interneuronal impulses. 6. EPSCs mediated by dorsal interneurons were smaller in amplitude (36.3 +/- 15.7 pA, mean +/- SD) than EPSCs mediated by medial interneurons (88.4 +/- 45.7 pA). 7. Paired-pulse facilitation of EPSCs was observed in PGNs (147.2 +/- 26.2%). The degree of facilitation was higher in dorsal (174.6 +/- 10.3%) than in medial interneuronal pathways (120.9 +/- 3.6%). Within each of interneuronal pathways the degree of facilitation was independent of the magnitude of the unitary EPSC. 8. The results show that PGNs receive monosynaptic glutamatergic excitatory inputs from two distinct populations of interneurons in the dorsal and medial regions of the SPN. These two populations of interneurons are likely to have different functions in the regulation of the preganglionic outflow to the pelvic organs.