It was well known that the presence of iodide had a negative effect on determination of mercury by the flameless atomic absorption photometry. In this case, however, a permissible amount of iodide differed from one another in workers. Then, the effect of iodide on the mercury determination was examined. As a result, up to about 1μg/ml iodide had no effect on the determination of up to about 0.01μg/ml mercury within the relative error ±4%. For the determination of mercury in tissues of rat the following method was applied. Tissues of rats were digested with perchloric acid-nitric acid mixture at 70-75°C overnight. The solution was diluted to 100ml with water, and then 5ml of sulfuric acid (1: 1) and 10ml of stannous chloride solution (10%) were added. 0.01-1μg of mercury was determined with 98-104% recovery in the presence of up to 100μg of iodide.