The clinical histories and pathological findings of six animals dying within a few days following bilateral constriction of the renal arteries have been presented. No explanation for the rise in blood pressure was found. The clinical histories and pathological findings of twelve animals with a sustained hypertension have been considered together with the detailed account of one of these in which a marked degree of arterial disease was found. It has been suggested from the study of this group that, though no cause for the initiation of the hypertension was found, the induced arterial disease might account for its maintenance.