Numerous circular fenestrations or pores were present in the pericardium of the rat, golden hamster, and mouse. The pericardial pores were most numerous in the mouse. They were usually less than 50 micron in diameter and situated in a meshwork formed by thick, wavy bundles of collagen fibers and thin, straight, elastic fibers. These pores directly connected the pericardial and pleural cavities and indirectly connected the right and left pleural cavities; i.e., the pericardial cavity in the rodents is not independent from but intercommunicates with the adjacent pleural cavities. There were many aggregates of free cells or milky spots in the pericardium. They principally consisted of lymphocytes, macrophages, and mast cells and were directly exposed to the pericardial cavity.