“…Since iodide (Wallace & Brodie, 1937, 1939, 1940aGildea & Man, 1943;Greenberg, Aird, Boelter, Campbell, Cohn & Muryama, 1943;Davson, 1955;Davson & Spaziana, 1959), sucrose (Hubbard & Zoll, 1949;Deane, Schreiner & Robertson, 1951;Gamble, Robertson, Hannigan, Foster & Farr, 1953;Davson & Matchett, 1953;Davson, 1955;Peterson, O'Toole, Kirkindall & Kemp-Thorne, 1959), inulin (McLennan, 1957;Morrison, 1959: Rothman, Freireich, Gaskins, Patlak & Rail, 1961, and serum albumin (Fishman, 1953;Rozdilsky & Olszewski, 1957;Van Wart, Dupont & Kraintz, 1960;Lee & Olszewski, 1960;Brown, 1961) have been used in various space measurements and barrier studies, and since they are relatively non-toxic substances, representinlg both charged and uncharged molecules and ions that have markedly different molecular weights (iodide 127, sucrose 342, inulin 3000-4000, albumin 69,000), they were selected for the present studies of transport and barrier mechanisms. In this investigation the tracers were administered to rats intraperitoneally (i.P.…”