Abstract: The present invention relates to a method for determining the selectivity of a pre-synaptic neuromuscular blocking substance for striated muscle or smooth muscle. The method comprises measuring the effect induced by a pre-determined quantity of pre-synaptic neuromuscular blocking substance upon electrically stimulating striated muscle tissue, measuring the effect induced by a pre-determined quantity of a pre-synaptic neuromuscular blocking substance upon electrically stimulating smooth muscle tissue, and calculating a quotient of the results obtained in order to determine the selectivity of the pre-synaptic neuromuscular blocking substance for striated muscle or for smooth muscle.