Serotonin, glutamate, glycine, ATP, and muscarine had no effect on resting membrane potential of muscle cell in earthworm Lumbricus terrestris. Nicotine depolarizes and GABA hyperpolarizes the muscle membrane. Removal of K(+), Cl(-) and addition of ouabaine and strychnine to the solution abolished the effect of GABA. The authors conclude that the Lumbricus terrestris myocyte membrane contains nicotine receptors and GABAergic receptors sensitive to strychnine. Stimulation of these receptors activates sarcolemmal ionic pumps and causes membrane hyperpolarization.