IN connection with some problems on the physiology of the heart under investigation in this laboratory, the question arose as to whether the readily extractible depressor substance was the base histamine.It is well known that simple aqueous or alcoholic extracts of almost any tissue cause a fall in blood-pressure on injection into the circulation of the cat. The heart is not an exception. In fact, it yields an extract with powerful depressor activity.The marked similarity in the physiological behaviour of tissue extracts and of solutions of histamine gave rise to the view that histamine was present in these extracts, a view which has been confirmed recently by the isolation of pure histamine, in the form of its dipicrate, from extracts of liver and lung [Best, Dale, Dudley and Thorpe, 1927], of muscle [Thorpe, 1928], and of spleen [Dale and Dudley, 1929]. It therefore seemed reasonable to expect that the depressor substance of heart extracts would be histamine. This expectation has, in fact, been realised.Apart from giving additional confirmation to the view that extracts of most tissues contain histamine, proof of the presence of histamine in heart extracts is of significance in connection with recent work on the so-called "heart hormone." Many workers have observed that heart extracts have a stimulating action on the frog's and rabbit's heart but there is much disagreement as to the nature of the substance responsible for these effects. Some workers [Demoor, 1924;Haberlandt, 1927] are of the opinion that the substance is formed in certain regions of the heart and is a specific heart hormone. Others, however [Rigler and Tieman, 1927;Rigler, 1929; Oppenheimer, 1929], deny the specificity of this substance, and have obtained extracts producing similar effects not only from any region of the heart but also from a variety of other tissues. Further, they have produced similar effects with pure histamine alone. Rigler, in fact, on physiological grounds puts forward the view that the substance is actually histamine. A full discussion of the work in this field is beyond the scope of this paper. The main features are briefly but effectively summarised by Oppenheimer [1929], who gives references to the more important papers.