By means of ultrasonic method, used in acute experiments on cats with closed abdominal cavity under nembutal narcosis, we studied the linear and volumetric blood flow velocity in the left phrenic artery, vascular resistance, systemic blood pressure, lung ventilation, arterial blood gases during different degrees of hypoxia and hypercapnia. It was shown that hypoxia and hypercapnia resulted in a decrease of the phrenic artery vascular resistance and an increase of the blood flow in the phrenic artery, not always proportional to hypoxia and hypercapnia degree. The correlation of an increase of the lung ventilation with an increase of the blood flow in the phrenic artery depends on the factor causing activation of the diaphragm performance. Some extreme conditions (prolonged asphyxia, blood loss, the exposure to 3% O2) lower phrenic vascular resistance, providing maximal blood supply of the diaphragm.