This study was undertaken to investigate the effect of lead on organisms. Mice received 15mg or 30mg of lead acetate per kg body weight every day for 1, 2 or 3 weeks, and the livers and kidneys were removed 24h after repeated injections. The livers and kidneys were used as sources for measurement of enzyme activities and for observation of alterations in ultrastructure. It was observed that body weights of mice treated with lead acetate were decreased when compared with those before treatment. This decrease in body weight was proportional to dose. The enzyme activities of succinate and malate dehydrogenases of experimental group that was treated with lead acetate for 1 week were nearly unchanged when compared with controls, but the enzyme activities of experimental group that was treated with lead acetate for 2 or 3 weeks were lower than those of controls. Changes in the enzyme activities were dependent on, but were not proportional to dose. Histologic examination of livers and kidneys after lead treatment showed that lead compound was accumulated and damaged in nucleus and mitochondria mainly. It was also observed that intranuclear inclusion bodies were formed only in epithelial cell of kidney proximal tubule after lead treatment. The overall changes in the ultrastructure were much greater in the livers than in the kidneys. From the above results, it nay be possible to conclude that the lead results in the decrease in body weight, reduction in the succinate dehydrogenate and malate dehydrogenase activities, and damages in the ultrastructure of kidney and liver in mouse. The presence of intranuclear inclusion bodies only in the kidney implies that these bodies protect the kidney from lead toxicity to some extent.