An experimental canine model was designed to measure directly the uptake, storage, and excretion of antibiotics by the liver and kidney. At equilibirum the rate of uptake of penicillin G, cephalothin, and nafcillin by these organs was about 80% of the rate of intravenous infusion of each antibiotic. Penicillin G and cephalothin were extracted mainly by the kidneys, and nafcillin by the liver. Injection of probenecid virtually abolished the difference in concentration of antibiotic between afferent and efferent vessels of the liver and kidney after 30-45 min. Renal tubular secretion of penicillin G and cephalothin was suppressed, and their levels in renal tissue were increased. These findings militate against any primary limitation by probenecid of access of antibiotic to the renal parenchyma. A marked sustained increase (60%-70%) in the rate of portal flow followed injection of probenecid, and the concomitant percentage of nafcillin extracted by the liver declined significantly. Because of the circulatory changes, a specific effect of probenecid on acess of nafcillin to the liver could not be proved.