1. The role of glutamate in slow excitatory synaptic transmission between small-diameter primary afferents and deep dorsal horn neurons was examined in neonatal rat spinal cord in vitro with the use of the whole cell voltage-clamp technique. 2. Single-shock electrical stimulation of large-diameter A beta-fibers evoked a short-latency (< 10 ms) fast (< 500 ms) excitatory postsynaptic current (EPSC). Stimulation of small-diameter A delta- and C fibers resulted, in addition, in a slowly rising and decaying EPSC (lasting up to 14 s) following the fast EPSC. The slow EPSC was never observed with stimulation of A beta-fibers. 3. Two patterns of EPSCs were observed, "type 1" and "type 2," which differed in their time course (lasting up to 1 and 14 s, respectively). The type 1 response was biphasic, with a fast monosynaptic component followed by an invariant, presumably monosynaptic, late slow component. The type 2 response was multiphasic, with a fast monosynaptic component followed by a slow component composed of fast polysynaptic currents superimposed on a slow current. 4. The fast monosynaptic component had a linear conductance, whereas the late slower component of the A beta-fiber-evoked response had a negative slope conductance at holding potentials more negative than -23 mV. Both currents reversed at a membrane potential of -1.2 +/- 2.8 (SE) mV. 5. With the use of selective non-N-methyl-D-aspartate (non-NMDA) and NMDA receptor antagonists [6-cyano-7-nitroquinox-aline-2,3-dione (CNQX) or 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo (F) quinoxaline and D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), respectively] we showed that both the early fast (A beta-fiber evoked) and the late slow (A delta- and C fiber evoked) components were mediated by non-NMDA and NMDA receptors. CNQX suppressed both the early fast and late slow components of the compound EPSC, whereas D-AP5 suppressed the polysynaptic currents of the early fast component and the late slow component without significantly affecting the early fast monosynaptic component. 6. Slow EPSCs summated on low-frequency (1 or 10 Hz), repetitive stimulation and produced long-duration "tail" currents on cessation of the stimulus. The amount of temporal summation was proportional to the duration of the slow EPSC and the frequency of stimulation. 7. Our results suggest that slow ionotropic-glutamate-receptor-mediated EPSCs produced by the stimulation of small-diameter primary afferents play an important role in activity-dependent synaptic plasticity in the dorsal horn.