The citogenetic lesions were evaluated in the marrow erythroblasts of 45 anesthetized white nonlinear male rats, weight--200-300 g who were subjected to an acute blood loss with a 1-hour arterial hypotension (ABR = 40 mm Hg); the micronucleus tests was made use of. Two stages of the increase of polychromatophilic erythrocytes with micronuclei in the marrow of the animals, who underwent a massive blood loss, were registered: stage 1--an incomplete marrow ischemia with a subsequent arterial hypotension and with a reliably confirmed formation of cytogenetic lesions in the marrow polychromatophilic erythrocytes; stage 2--the reperfusion period contributed to a 1.7-fold increase of polychromatophilic erythrocytes with micronuclei versus the previous stage. Mexidole, when used at 50 mg/kg prior to blood reinfusion, decreased the quantity of polychromatophilic erythrocytes with micronuclei to the basic level, which is indicative of reversibility and instability of cytogenetics impairments in the marrow cells of animals observed in the early post-resuscitation period.