Abstract: Experiments were carried out on the isolated rat phrenionerve dia-phragm preparations and chick biventer cervicis preparations. Cu²⁺ at cumulative concen-trations inhibited muscle contractile response to indirect stimulation without affecting theconductivity of the nerve. The inhibiting action of Cu²⁺ on the muscle contractile responseto indirect stimulation was stronger than that to direct stimulation. Cu²⁺ did not affect theAch sensitivity of the chick biventer cervicis muscle. The effects of Cu²⁺ on neuromusculartransmission were antagonized by Ca²⁺ and L-cysteine. It is concluded thatCu²⁺ produces neuromuscular blockade by acting mainly on the presynaptic nerve termi-nals, binding with the -SH groups of the membrane and thus altering thetransmembrane movement of Ca²⁺ and reducing the release of Ach from the nerve termi-nals.