AbstractFive‐hydroxy tryptamine (5‐HT) causes a hyperpolarization and increased conductance of the leech body wall muscle cell membrane. If 5‐HT is applied in the absence of the Cl−ion, the response appears as a depolarization, whereas if 5‐HT is applied in the absence of the K+ion, the response is a hyperpolarization. In both cases, the conductance of the muscle cell membrane is increased. Stimulation of the peripheral nerve to the body wall muscle produces a complex junctional potential in muscle cells. Exposing the muscle tod‐tubocurarine (d‐TC) eliminates the excitatory component (EJP) of the complex potential. The inhibitory potential (IJP) that remains has an equilibrium potential at approximately 65 m V. Furthermore, this IJP appears as a depolarization when the nerve is stimulated in the presence ofd‐TC and low CL−, whereas this is not the case if the nerve is stimulated in the presence ofd‐TC and low K+. The drugs BOL‐148 and cyproheptadine block the IJP's in the body wall muscle. These data are interpreted as indicating that 5'HT acts on leech body wall muscle cells by increasing the conductance to the Cl−ion and that the IJP's caused by nerve stimulation are probably the result of 5‐HT release at nerve terminals.As a final point, it has been shown that the inhibition by 5‐HT of the spontaneous EJP's that occur on the leech body wall muscle results from an inhibition of central neurons and not from any direct effect on the muscle cel