Some techniques to block muscular nerve evoked contraction involve pharmacological approaches using synaptic blocking agents. Such methods interfere with normal synaptic transmission, and could introduce artifacts making difficult the experimental interpretation. The method based on the use of formamide pre-treatment should not interfere with synaptic physiology, indeed previous works suggest that the mechanism involved in block of muscle activity could depend on the decrease in specific postsynaptic membrane capacitance, and on the disruption of the morphology of the transverse tubule system. To prove this assumption we evaluated before and after formamide pre-treatment, some pre and postsynaptic parameters related to the spontaneous quantal release (MEPC). By means of the Loose patch clamp technique, we demonstrated, that formamide pre-treatment increases in an irreversible manner the frequency of spontaneous quantal release. Morphology of MEPC appear not modified by formamide pretreatment, which does not interfere with postsynaptic cholinergic receptors activity.