We analyzed glutamate-induced enhancement of the amplitude of monosynaptic excitatory postsynaptic potentials evoked by stimulation of the lateral column fibers (LC-EPSPs) on lumbar motoneurons in the frog spinal cord. Low concentrations (0.1-0.3 mM) of glutamate, which produced small depolarization, often enhanced EPSP associated with inhibition of a paired pulse facilitation and increased occurrence of spontaneous EPSPs. With 1 mM glutamate, transient enhancement of EPSP was seen in some cells during the early phase or prior to large depolarization, even when input conductance was increased. Transient or sustained enhancement of EPSP was occasionally seen with N-methyl-D-aspartate, kainate and quisqualate, but not with L-2-amino-4-phosphonobutyrate. The results suggest that glutamate enhanced release of excitatory transmitters at low concentrations that apparently did not affect the postsynaptic membrane.