The routes by which radioiodinated serum albumin placed in the pericardial cavity gains access to the circulation have been investigated in rabbits. 1. Eighty per cent of pericardial cavity protein passes through the parietal pericardium and into the circulation through the thoracic duct. 2. A small amount of protein is drained through the right lymph duct; this is probably derived from protein passing from the pericardial cavity into the pleural cavity. 3. There is no apparent movement of protein directly into blood vessels of the parietal pericardium. For theoretical reasons movement of protein across the visceral pericardium and into the blood vessels of the myocardium is also unlikely. 4. A small amount of protein enters the circulation when both major lymphatics are ligated. It is proposed that lymphatic uptake may continue and secondary lymphovenous junctions will open as a result of raised intralymphatic pressure.