In conscious dogs, arginine vasopressin (AVP) inhibits an angiotensin II drive to ventilation during air breathing and during acute hypercapnia. To determine whether AVP inhibits respiration in rats, as in dogs, respiration and metabolism were measured in six male Sprague-Dawley rats using a plethysmograph. Rats breathed air, followed by 5% and 6.5% CO2 with or without AVP V1 receptor block. In unblocked experiments, minute ventilation (V) increased to a comparable level during inhalation of both CO2 gas mixtures, resulting in a flattening of the ventilatory response to increased Paco2. However, oxygen consumption decreased during 6.5% CO2, compared with 5% CO2, so that the ventilatory equivalent for O2 increased in a more linear manner with respect to Paco2. The main effect of AVP V1 receptor block was to increase mean arterial blood pressure; there was no significant effect of AVP V1 receptor block on respiratory responses. AVP does not inhibit respiration in conscious rats as it does in conscious dogs.