Synaptic responses induced in motoneurones by stimulation of dorsal root (DR) and reticular formation (RF) were recorded intracellularly in the isolated frog spinal cord. Argiopine (spider neurotoxin, a selective blocker of glutamate receptors of the non-N-methyl-d-aspartate (non-NMDA) type) in concentrations from 3 × 10^-7 to 1 × 10^-5 M effectively inhibited di- and polysynaptic components of excitatory postsynaptic potentials (EPSPs) induced by DR and RF stimulation. The monosynaptic component of the RF response was inhibited by argiopine, whereas the monosynaptic component of DR response was actually enhanced. All types of EPSPs studied were blocked by kynurenate (1ndash;2 × 10^-3 M). d,l-2-Amino-5-phosphonovaleric acid (1 × 10^-4 M slightly diminished the amplitude only of polysynaptic DR responses. A special type of argiopine-resistant, non-NMDA glutamate receptor is assumed to be involved in the generation of an EPSP by the monosynaptic DR input.