The effects of beta-agkistrodotoxin (beta-AgTX) on synaptic transmission of the toad sympathetic ganglia were investigated by intracellular recording techniques. Superfusion of beta-AgTX (30 microgram/ml, 5-15 min) reversibly inhibited the cholinergic fast excitatory postsynaptic potential (f-EPSP, n = 16) and the fast components of acetylcholine (ACh) potential induced by micropressure administration of ACh (n = 24). Comparison of beta-AgTX effect in the same cell group showed significantly different inhibition rates on f-EPSP (77.2 +/- 27.7%) and ACh potential (25.5 +/- 17.5%) (n = 6, P less than 0.01, F test). During application of beta-AgTX (30 or 50 micrograms/ml) for 15-30 min, no detectable change was found in non-cholinergic late slow EPSPs (n = 22). The results suggest that beta-AgTX selectively inhibits the cholinergic transmission of the toad sympathetic ganglia by both presynaptic and postsynaptic mechanism.