Anesthetized dogs were artificially cooled to temperatures at which a marked prolongation of systole was observed. Under these conditions, it was possible to identify undulations in pressure during the prolonged systole without confusion with incisural pressure changes. These pressure undulations in the aortic arch were observed to occur simultaneously with but in opposite phase relations to the primary peak and dicrotic oscillations recorded from the femoral artery. This evidence offers additional support to the standing-wave theory.