The first part of this paper is concerned with the distribution of organo-chlorine pesticides in mice using a liquid scintillation counter, and an attempt was made to make clear its mode of action. (14)C-DDT and (14)C-Dieldrin were given orally to mice. It was found that after 24 hours, both compounds were taken up by all tissues examined (liver, kidneys, heart, lungs, spleen, brain, with high concentrations in fat tissues). Their concentrations gradually increasd, reaching the peak levels after 48 hours and therafter gradually decreased, but even after 30 day a small amount of these compounds was still, detectable in most organs, especially in fat tissues. Since these compounds once deposited in tissues, remain unexcreted for a long period of time, much attention should be paid to prevent their chronic intoxication.