Female albino rats were exposed to carbon monoxide in concentrations ranging from 130 to 1030 mg/m3 for a period of 40 minutes. During the last 20 minutes of exposure, the rats were subject to forced run on a flat treadmill moving at a rate of 400 m/hour. Immediately after termination of exposure, the concentration of carboxyhemoglobin and lactic acid in blood, as also the concentration of carboxymyoglobin in the heart and skeletal muscles were determined. Work load did not influence the level of carboxyhemoglobin in blood, though inducing an increase in the concentration of carboxymyoglobin in both the heart muscle and the skeletal muscle. The increase in the concentration of lactic acid in the blood was observed only in rats exposed to carbon monoxide in an average concentration of 1030 mg/m3 and simultaneous work load. Results of the observations demonstrate that simultaneous effect of carbon monoxide and work load can induce more severe tissue hypoxia than could be expected on the basis of carboxyhemoglobin concentration in the blood.