The effects on respiratory rate and amplitude following intravenous injection of the vasodilator drugs, Urecholine (carbaminoyl-beta-methylcholine) and nitroglycerine were determined in dogs before and after bilateral vagotomy and isolation of the carotid sinuses. The animals were anesthetized with sodium pentothal followed by alpha-chloralose (70 mg/kg). Following the procedures for denervation of sino-aortic pressoreceptors and carotid and aortic chemoreceptors the animals showed no respiratory response to 1–2 mg sodium cyanide. It was found that the respiratory stimulation which occurs during the fall in blood pressure induced by injection of Urecholine or nitroglycerine frequently persists after sino-aortic denervation. The respiratory stimulation in the denervated animals in response to nitroglycerine was reduced or prevented in individual experiments by counteracting the fall in arterial blood pressure by means of a pressure-regulating device. It is concluded that in the dosage used nitroglycerine, and possibly also Urecholine, exert their effects on respiration indirectly by causing alterations in blood pressure or blood flow rather than through any specific action on the respiratory center.