Green patina, a kind of rust which forms on the surface of copper, has traditionally been suspected of being virulently poisonous, so the Food Sanitation Law of Japan prescribes that kitchen utensils and tableware made of copper must not have any green patina on the surface. However, little is known about the toxicity. In order to gain more insight into the toxicity, we have initiated a study into the toxicological effect of basic cupric carbonate in the diet on rats. Rats were fed a diet containing basic cupric carbonate, which is the major component of the patina, at doses of 0, 70, 220, 670 and 2, 000ppm as Cu(OH)2·CuCO3·H2O. After feeding for 1, 3, 6 and 12 months, the levels of copper in the blood and tissues were determined by atomic absorption analysis. At the same time, fluctuations in the levels of iron and zinc, which are essential elements, were determined in the tissues.Levels of copper, iron and zinc in blood were practically unchanged after feeding for 12 months at the highest dose, 2000ppm. Copper in the liver increased markedly at doses of 670 and 2000ppm from 1 month, and the level reached about 50 times that of the control group. The tendency was more marked in females than in males. The level of copper in the kidney significantly increased at larger doses than 670ppm.Iron concentration in the kidney did not change at smaller doses than 220ppm, but significantly decreased at higher doses than 670ppm. The level of copper in the kidney increased only at the highest dose.In conclusion, basic cupric carbonate, is not as toxic as formerly believed, and the levels of essential metals in tissues were not affected by feeding at smaller doses than 670ppm in the diet for 12 months.