The location of chemoreceptors causing diving bradycardia in bullfrogs, Rana catesbeiana, was investigated. Intravenous sodium cyanide injections (50–75 μg/kg) into unanesthetized frogs caused hyperventilation and an increase in arterial blood pressure. When frogs were submerged, such injections produced bradycardia and a fall in blood pressure.Injections of cyanide (25–50 μg/kg) into the right or left pulmocutaneous arch of lightly anesthetized frogs which were breathing acted as a respiratory stimulant; similar injections produced bradycardia when frogs were submerged. Injections of cyanide into the right or left carotid arch when frogs were breathing stimulated ventilatory movements. Injections into the carotid arch when frogs were submerged had no substantial effect on cardiac rate. No responses were noted after cyanide was introduced into the right or left systemic arch.These preliminary data suggest that there are respiratory chemoreceptive areas in bullfrogs which are stimulated by blood perfusing both the carotid and pulmocutaneous arches. Chemoreceptors located along the circulation of the pulmocutaneous arch may play a role in the bradycardia response during submergence in bullfrogs.