The effect of intravenous injections of adrenaline on the cat's paw vessels has been investigated with a special type of optical plethysmographic method. The method is described. Changes in the paw volume are considered to be mainly, if not solely, due to changes in the skin vessels.In the animal kept warm, under normal laboratory conditions, adrenaline from the smallest effective to the largest applied doses always produced a constriction of the paw vessels, irrespective of the effect of adrenaline on the blood pressure, which may be pressor or depressor. A marked constriction of the paw vessels is seen after doses which do not alter the blood pressure.Since the blood pressure may reach a considerable height, or even its maximum, before any constriction can be detected, and since a constriction of the paw vessels is seen even if the blood pressure falls, it is concluded (1) that the reaction of the skin vessels does not play such a deciding part in producing the first elevation of the blood pressure after adrenaline injections as is usually accepted, and (2) that adrenaline acts much more in the sense of a blood distributor than blood‐pressure augmentor. This is confirmed by the effect of adrenaline injections in the cooled animal, where the constriction of the paw vessels is suppressed and an increase in paw volume can be registered. It is argued that the functional state of the paw vessels is responsible for the change in the reaction, and that the adrenaline reaction is subordinate to those reactions on the skin vessels which control the body temperature, as already recognised for the muscle vessels.The increase in paw volume which has been observed under certain conditions is neither increased by eserine nor abolished by atropine; hence it is not cholinergic in type. It is, however, suppressed by ergotamine. The probable underlying physiological reaction in the increase in paw volume is discussed.All the reactions registered by the plethysmograph have been confirmed by the observed changes in the skin temperature.Part of this research was carried out while the author was in the Department of Physiology, Edinburgh. The author is much indebted to Professor I. de Burgh Daly for his hospitality, constant advice, and the interest taken in this work.It is with pleasure that I put on record my indebtedness to Professor C.F.M. Saint for his interest and constant help.The expenses of the research have been defrayed by the Marais Fund, for which help I express my thanks.