Argiopin, a low-molecular weight component of the spider Argiope lobata venom, inhibited depolarizations of motoneurones induced by glutamate in experiments on the frog isolated spinal cord, but had no effect on aspartate-induced responses. Half of the blocking effect (ED₅₀) was seen at 7.5 ± 3.7 × 10^-8 M argiopin. The same concentrations of argiopin (7.5 × 10^-8 M to 2.3 × 10^-7 M) suppressed the responses of the ventral root to electrical stimulation of the dorsal root. The results suggest that argiopin selectively blocks only one population of the excitatory amino acid receptors on motoneurones, and these argiopinsensitive receptors are found to be involved in sensorimotor synaptic transmission in the spinal cord.