The first randomized double-blind placebo-controlled study of efficacy of mexidol has been conducted. Fifty-one patients with ischemic stroke, aged 45—85 years, admitted during the first 24 hours after stroke were switched to mexidol in dosage 300 mg daily during 14 days. Twenty seven patients received the placebo using the same scheme. The significant forward dynamics in regress of neurological disturbances on the NIH scale was found to the 14th day in patients treated with mexidol as compared to the placebo group (p<0.05), along with functional rehabilitation assessed according to the dynamics of clinical scores on the Bartel scale to the 21st day after stroke (p<0.05) in patients involved in the study during the first 6 hours after disease onset. The normalization of functional brain activity was revealed in patients receiving mexidol, in comparison with the placebo, i.e. during the acute pharmacological test. A study of the antioxidant system demonstrated the increase of enzyme activity (superoxide dismutase, glutathione peroxidase, glutathione reductase) and the analysis of enzyme activity of the respiratory mitochondrial chain revealed the increase of succinate dehydrogenase activity in patients receiving mexidol. The results confirm the clinical efficacy of mexidol in the acute period of ischemic stroke in the carotid system of atherothrombotic or cardioembolic pathogenetic types, especially for its earlier (up to 6 hours) prescription, and specify mechanisms of its action (antihypoxic antihypoxic, antioxidant) in clinical conditions.