The present report analyses the blood-pressure action of dihydroergocornine (DHO) in anaesthetized animals without artificial hypertension. Experiments are carried out on intact and nephrectomized animals in cats in which the renal arteries are temporarily clamped, in cats with section of the thoracic spinal cord, and in dogs with grafted kidney. Evidence is presented that the changes in blood-pressure following dihydroergocornine in the intact animal are the result of at least two interacting mechanisms. The one (leading to a depressor effect) is only apparent if certain nervous mechanisms are intact, while the second (leading to a pressor effect) depends on renal function. It is probable that these effects on the circulatory system are produced by different active principles. Experiments in dogs with grafted kidney show that the secondary blood-pressure effect following dihydroergocornine is caused predominantly, to say the least, by means of a humoral mechanism of renal origin.