AbstractIn mice aggressive behaviour causes a vast release of renin, which can result in about 600‐fold increase in plasma renin concentration, reaching 6 Goldblatt Units, corresponding to 15μg renin per ml. This increase is mainly due to release of submaxillary renin, but there is also a significantly increased renal renin release. The degree of renin release is influenced by the duration of the aggression and by previous contact with other mice. Contrasting with the vast increase in plasma renin the blood pressure is normal or only moderately increased. This disproportion is not due to the depletion of renin substrate, caused by the increased renin, as shown by the increased calculated renin activity, as well as by decrease in blood pressure elicited by blockade of the renin system. Nor is the disproportion due to change in the sensitivity of the vessels to angiotensin II, the cause of this lack of tachyphylaxis being unknown. By way of exclusion the lack of pronounced increase in blood pressure can be explained by homeostatic function of the cardiovascular reflexes, which may also account for the fact that the pressor response after injection of pure submaxillary renin is only short, contrasting with a prolonged marked increase in plasma renin concentrati