Argiotoxin 636, (AR 636), a synaptic antagonist from orb weaver spider venom, is shown produce reversible blockade of excitatory transmission in CA1 pyramidal neurons of the in vitro rat hippocampus. Microtopical applicationof AR 636 (5–50 nM) resulted in a concentration-dependent suppression of the amplitude of the dendritic field EPSP recorded from stratum radiatum, and the amplitude of the population spike recorded from stratum pyramidale in response to stimulation of the Schaffer collaterals. The maximum effect of AR 636 occured at about 15–25 min. These effects were reversible after washing with toxin-free physiological solution with the rate of recovery having an inverse relationships to the concnetration of AR 636. In contrast to the effects observed with orthodromic stimulation, the amplitude of the antidromic spike was not affected by exposure to AR 636. The temporal pattern GABAergic paired-pulse inhibition was unaffected by exposure to AR 636. Neuronal discharge elicited by pressure injection of l-glutamate was abolished by AR 636, whereas, responses to l-aspartate were not significantlu affected. These data suggest that AR 636 functionsas a selective antagonist of glutamate-mediated synaptic transmission in rat hippocampus.