The mechanism of binding of toxin MT2 from venom of green mamba Dendroaspis angusticeps to muscarinic acetylcholine receptors from rat cerebral cortex was investigated by studying the kinetics of the toxin-receptor interaction. The muscarinic antagonist N-methyl-[3H]scopolamine was used as a 'reporter' ligand. Evidence for a mechanism of toxin-receptor interaction comprising at least two steps was obtained. Such a mechanism increases the potency of the toxin. The first step was fast with no competition between the toxin and the antagonist. The second step was slow with formation of a more stable toxin-receptor complex and inhibition of the antagonist binding. It is proposed that the snake toxin is a muscarinic agonist of slow action.