Air infused into the jugular vein of dogs at a constant rate for one hour produced transient hypotension, modest elevation of the systemic venous pressure and a considerable fall of the arterial oxygen tension. All of these measurements returned almost to normal on the termination of the infusion. A lethal rate of infusion, 0.69 ml of air per kg of body weight per minute, produced heart failure with a decided rise of systemic venous pressure and fall of blood pressure. The pre-failure oxygen pressure did not fall to a lethal level. Six and a half minutes was the longest time that the characteristic murmur could be heard after the cessation of infusion of non-fatal doses of air.