The cardiovascular responses to hypoxia (oxygen tension in inspired water, PiO2 = 30-40 mmHg) was investigated in the cod before and after bilateral sectioning of the glossopharyngeal (IX) and the pretrematic ramus of the vagus (X) innervating the first pair of gill arches. The study demonstrates in the cod two distinct mechanisms of responses to environmental hypoxia. The first mechanism involves oxygen receptors located in the areas of the gills innervated by the nerves sectioned in the experiments. Hypoxic stimulation of these receptors produced slowly (approximately 6 min) developing bradycardia, and despite a 30%-50% decrease in heart rate, cardiac output remained virtually constant due to a concomitant increase in cardiac stroke volume. The second mechanism involves oxygen receptors located elsewhere, and these receptors trigger a rapidly (within approximately 2 min) developing increase in both ventral and dorsal aortic blood pressure, caused by an increase in the systemic vascular resistance.