Experiments in situ on mature white female rats performed with the use of electrophysiological methods allowed to investigate the modulatory influence of thyroxin at the dose which does not cause the signs of hyperthyroidism (10 mkg/ kg), upon the manifestation of the dexamethasone effects on the functional state of the anterior tibial muscle. It has been established that the chronic isolated application of dexamethasone was accompanied by reduction of the amplitude of muscle contraction (by 29.7–59.3 per cent after 10–50 days of the drug injection) and the weight of anterior tibial muscle (by 22.4–12.7 per cent after 10–60 days of the drug injection). Combination of thyroxin with dexamethasone smoothed the negative effects of the synthetic glucocorticoid upon the muscle contraction amplitude and even caused its increase (by 41.2–62.1 per cent after 20–60 days of injection of the pair of preparations), as well as prevented the reduction of the muscle weight. The isolated application of dexamethasone after the first 20 days of injections caused the decrease of the muscle speed that was confirmed through a lengthened control of the muscle active state duration (by 20.5 per cent) and the reduction of its single contraction development speed (by 45.3 per cent), as well as the decrease of frequency of muscle tetanization (to 12-20 imp/s against 26-28 imp/s in control). The application of thyroxine with dexamethasone shortened the active state of the muscle (by 19.3 per cent) and increased the speed of single contraction development (by 72.4), which remained throughout whole further period the preparations were injected. These observations favor for improvement of high-speed characteristics of the muscle under the influence of thyroxine. At the same time, during chronic injection of dexamethasone either alone or in combination with thyroxin, an increased muscle fatigue during the first 10–20 days has been observed.