The permeability properties of skeletal muscle from Periplaneta americana and Sphinx ligustri have been studied by several methods and in each the effects of pharmacological agents and of temperature have been investigated. The properties of the microsomal preparation of skeletal muscle were characterized. Compared with preparations from many other types of excitable cell, although probably not muscle, the microsomal preparation was a typical in lacking an enzyme synergistically stimulated by monovalent cations and in being insensitive to strophanthin G. Normal trans-membrane inorganic ion distributions of skeletal muscle were determined. The ratios were similar in the two animals and not markedly dissimilar from those of vertebrate muscle / blood ratios. The results obtained with Sphinx muscle differed from those of previous workers. Based on the results obtained a new saline was developed for bathing the muscles of Periplaneta and Sphinx which proved to be more suitable than salines used by other workers. Intra cellular microelectrode recordings were made from resting and active skeletal muscle of Periplaneta. The relative contributions to the resting membrane potential of actively and passively distributed ions were determined. A considerable hyperpolarizing contribution to the resting membrane potential was made by a metabolically dependent constant current generator. Similar results were obtained using Sphinx skeletal muscle. An electrical model was derived to account for the phenomena observed. The individual contributions of inorganic ions to depolarizing electrogenesis were determined. Sodium was found to make a major contribution. The possible role of electrogenic pumps i n the development of end-plate potentials was discussed. The excitatory postsynaptic potential of Periplaneta skeletal muscle was compared with the end-plate potential of vertebrate skeletal muscle and the possible nature of the biphasic response of Periplaneta muscle to excitation discussed. Possible mechanisms of excitatory and inhibitory eleotrogenesis were related to the model developed to describe the resting membrane potential.