The invention relates to a method for determining the quantity of pre-synaptic neuromuscular blocking substance (notably botulinum toxin) contained in a sample. In one aspects, the method comprises the following steps: (i) determining the minimum voltage Vm needed to induce the contraction of muscle tissue, said muscle tissue being connected to an electrical stimulator through a motor nerve and preferably immersed in an oxygenated physiological buffer containing glucose; (ii) adding the sample containing the pre-synaptic neuromuscular blocking substance; (iii) electrically stimulating, at a voltage at least equal to Vm, the muscle tissue at certain time intervals; (iv) comparing the effect induced by the sample to the effect induced by a reference substance and thereby determining the quantity of the pre-synaptic neuromuscular blocking substance in the sample.