The nature of low-molecular-weight proteinuria, which were observed in rabbits given nephrotoxic compounds such as cadmium, kanamycin, uranium, mercury or chromium, were studied in the present experiment : rabbits were given subcutaneous injections of cadmium 6 times a week at a dose level of 0.5 mgCd/kg over a period of 32 weeks. Main components of proteins in urine of cadmium-treated animals were proteins of molecular weight 67, 000 (probably albumin) and higher-molecular-weight, and that might suggest the existence of tubular dysfunc-tions as well as of glomerular dysfunctions. Rabbits given a single injection of uranium, chromium or mercury indicated low-molecular-weight proteinuria as well. Molecular weight distribution of proteins in urine of the uranium-, chromium-or mercury-treated rabbits were hardly distinguishable from that of cadmium-treated rabbits. Low-molecular-weight proteinuria appeared in spite that tubular reabsorp-tion of phosphorus was higher than 80%. The mechanism of low-molecular-weight proteinuria was discussed.