In the experiments on rats, the bone blood flow may be influenced by the deficiency as well by the administration of sex hormones. The aim of this work was to investigate the bone circulation (determined by means of 85Sr-microspheres, experiment A and B) and the incorporation of 45Ca and 3H-proline into the tibia (experiment C) in the last stage of pregnancy (from 18 to 21 days), i.e. in the physiological condition with elevated level of estrogens. In the experiment A and B, the uptake of 85Sr-microspheres and blood flow in pregnant rats was lower in the tibia and distal femur, the blood flow only was significantly lower also in the humerus and the calvaria. In kidneys, liver, muscle and skin, there was no difference in microsphere uptake; the blood flow was in pregnant females lower only in the kidneys. In perirenal fat, the microsphere uptake and blood flow was several times higher in pregnant rats than in controls. 24 hour incorporation of 45Ca and 3H-proline was in pregnant females significantly lower. The bone density and ash weight was higher in pregnant rats in experiment A. Estradiol concentration in plasma was in experiment B in pregnant rats significantly higher (by 24.8%). Thus, in pregnant female rats, we found lower bone blood flow and lower incorporation of 45Ca and 3H-proline along with normal or higher bone mineral content in the condition, where enhanced resorption and formation of the bone had been described. The changes in bone blood flow and possible role of estrogens in this situation remain to be elucidated.