Estradiol-<14>^C was injected into the dorsal aorta of goldfish (Carassius auratus) and the distribution of the isotope has been investigated autoradiographically. The estradiol-<14>^C was rapidly taken up from the blood by various tissues. The liver, the main steroid-metabolizing organ, showed the highest uptake among all tissues throughout the observation time. The accumulation of radioactivity by the gills and kidney which observed for the first 10 minutes after injection (Fig. 1) indicates the excretion of radioactive materials through these organs. The isotope was also excreted through the gall-bladder as 20 minutes after injection and later on a very high concentration was observed in the bile and intestinal contents. Apart from organs described above, selective accumulation was observed in the interrenal gland and gonads which are believed to be target areas of estrogens. There was also appreciable accumulation of <14>^C in the central nervous system and retina (Figs. 1 and 3). It is still open question, however, if this accumulation is in any way associated with physiological functions of estrogens. Sixteen hours after injection, no activity could be found in any tissue but the interrenal gland, gall-bladder, intestine, and liver (Fig. 6).