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Digitalis is one of the oldest and most widely used drugs in clinical medicine. Its action in congestive cardiac failure has been thoroughly studied by many workers. Its action upon the normal circulation, however, has received relatively little attention. The concept that it lowers cardiac output and reduces central venous pressure in normal subjects continues to be expressed by many workers. This view, based on studies employing older, often inadequate methods, has been neither confirmed nor disproved by means of existing hmmodynamic techniques. This study, utilizing the technique of cardiac catheterization, was undertaken for the purpose of investigating the heemodynamic effect of digoxin upon the circulation in 12 normal subjects. MATERIALS AND METHODSAll twelve patients were carefully investigated by clinical, electrocardiographic, and radiological methods, and in none was there any evidence of significant cardiovascular abnormality. Sedatives (0X1 g. sodium pentobarbital) were administered to five patients prior to the study. Special care was exercised that all measurements should be performed during a suitable post-absorptive steady state: that this was achieved is indicated by the small variation in duplicate control measurements prior to the administration of digoxin. Cardiac catheterization was performed in the usual manner. A branch of the basilic vein was dissected free under local procaine anwsthesia and a large size (No. 9F) cardiac catheter was introduced and guided under fluoroscopic control into the right ventricle and pulmonary artery. Pressures were recorded using strain gauge transducers and a direct writing oscillograph (Sanborn). The tip of the catheter was then withdrawn to the centre of the right atrium. For the remainder of the study frequent or continuous records of right atrial pressure were obtained. The sensitivity of the system was adjusted so that a deflection of 1 cm. on the tracing was equal to 1 mm. Hg pressure change. The zero reference point was midway between the recumbent patient's spine and the sternum. The reference point was rechecked at frequent intervals during the study. With the catheter tip in the right atrium, and when the patient had reached a satisfactory steady state after 15 minutes resting period, two control determinations of the cardiac output were made with a 15 to 20 minute interval between them. Oxygen consumptions were obtained by analysis of duplicate samples for oxygen and carbon dioxide in the patient's expired air collected in a Tissot spirometer during a three-minute period. Midway during the air collection period simultaneous samples were withdrawn from the right atrium and brachial artery, the latter through an indwelling arterial needle. Duplicate analyses for oxygen and carbon dioxide were performed using Van

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