In order to evaluate the nephrotoxicity by antibiotics, effects of these agents were investigated in terms of protein synthesis and nucleic acid metabolism in the kidneys of Wistar rats. Aminoglycoside antibiotics (streptomycin, kanamycin, gentamycin), tetracycline (doxytetracycline), chloramphenicol, and cephems (cephalothin, cephaloridine, ceftezol, latamoxef) were used. These antibiotics were given to the rats for 5 successive days. On the 6th day 14C-6-orotic acid and 14C-1-leucine were administered intraperitoneally. Incorporation rates of these tracers into ribonucleic acid and protein fraction in rat kidney ribosomes (polysomes) were measured. Another experiment was undertaken in vitro by separating the polysome fraction from the rat kidneys. In vitro acellular protein synthesis using these polysomes was established. Effects of antibiotics on the incorporation rates of 14C-1-leucine were examined. Marked reduction of incorporation of these tracers into nucleic acid and protein in vivo and in vitro was induced by aminoglycoside antibiotics. These facts suggest that aminoglycoside antibiotics develop nephrotoxicity by interfering with the metabolism of ribosomal RNA and protein.