The following experiments were carried out with the object (1) of studying the effect of a rapidly growing neoplasm on the metabolism of the normal animal which bears the tumour, with consideration also of the view that the new growth secretes, as has often been asserted, substances having a deleterious action on the tissues of the animal bearing the tumour; (2) of elucidating the processes determining the rapid proliferation of the cells of a malignant new growth, and the source of the nitrogenous material used by the tumour. In order to obtain facts throwing light on these questions, we have determined the nitrogenous metabolism in three rats before and after implantation of a rapidly proliferating malignant new growth. So far as we are aware, no such experiments have been made before.