Studies of the simultaneous rates of transfer of various tracers and stable isotopes from the pericardium of normal dogs under pentobarbital anesthesia indicated that the rate of transfer of Rb86was greater than that of Na24and Cl36, which were approximately equal. These observations show the pericardium to be a very active serous membrane concerned with electrolyte transfer phenomena as well as an excellent experimental model for the study of the kinetics of exchange of electrolytes across a normal living membrane.