In the experiments on female rats of Wistar strain the effect of metronidazole (Entizol - Polfa) was studied administrated in aqueous solution with a gastric pipette into the stomach. Dependence was analyzed on the dose of metronidazole, on the time interval of administration before irradiation and on the degree of hypoxia of bone marrow during irradiation induced in the lower part of the body and graded by the abdomen compression with an inflatable rubber cuff with overpressure 4, 6.7, 13.3 and 20 kPa. The effect of whole body irradiation (60Co source) was evaluated according to the changes of bone marrow cellularity (BMC) the third day after irradiation. The sensitizing effect of metronidazole increases rapidly up to dose of 30 mg/kg with rising dose of the drug, then at higher doses only slowly. With prolonging interval of the application of 600 mg/kg body weight the sensibilization increases and reaches the peak after 1-2 hours, then it gradually decreases till the complete disappearance after 10-15 hours. In both cases irradiation was carried out at abdomen compression corresponding to 20 kPa of overpressure in the rubber cuff. The induced hypoxia of bone marrow demonstrated a significant, protective effect, corresponding to the degree of abdomen compression which was reduced by metronidazole administration in the dose of 600 mg/kg. The reduction represents one half to one third of the protective value and is proportional to the degree of induced bone marrow hypoxia during irradiation.