Circulation time from an arm vein to the femoral artery as determined with I131 averaged 11.2 seconds in normal male adults. In the presence of heart failure the time of initial arrival was prolonged, and the slope of ensuing rise flattened. Decholin time, determined simultaneously, averaged 25 per cent longer than I131 time in both normal subjects and patients with congestive failure. Theoretical considerations indicate that arrival time measured with a radioactive tracer is less affected by cardiac dilatation than is arm to tongue time obtained with Decholin. Arm to heart time measured with I131 was prolonged in the presence of failure but marked overlap with normal subjects was evident.