The distribution in mice of 14C- and 3H-diethylstilboestrol has been investigated autoradiographically. The results have been compared with those which have been previously reported for natural estrogens. Many similarities have been demonstrated between the synthetic and natural compounds. Thus a specific accumulation has been observed in the endometrium, the granulosa layer of large ovarian follicles, the adrenal cortex, the interstitial tissue of the testes, and the hypophysis. Natural and synthetic estrogens differ widely concerning the penetration into and the distribution within the fetus.