The objective of this research is to identify the mechanisms of regulation of intracellular oxygen tension in muscle under a variety of physiological conditions. A method for the determination of intracellular oxygen tension in muscle, based on the spectrophotometric determination of the oxygen saturation of myoglobin was developed. This method will be used to determine oxygen tension in muscle at rest in difference oxygen tension in muscle at rest in different muscles with varying functions and under the following conditions: during rhythmic and sustained muscular contraction, during reduction in blood flow and subsequent reestablishment of the circulation, during reduction of the arterial oxygen tension induced by ventilation of the animal with gas mixtures containing reduced oxygen concentrations, during hypotension induced by hemorrhage, during stimulation of the sympathetic nerves supplying the vascular bed of the muscle under observation, during stimulation of the sympathetic cholinergic vasodilator system, and during administration of vasoactive substances.