[en] We have studied the distribution of 17β(16α-[¹²⁵I]iodo)-estradiol (I-E₂) in tumor-bearing and normal rats. High early adrenal-to-blood ratios (up to 22 at 5 min) were seen in all groups, but this fell to six at 1 h. Uterus-to-blood ratios of 15 were found, and these were fairly constant up to 2 h after administration. Uptake of label in the uterus, but not in the adrenals, was sensitive to excess diethylstilbestrol, which competes with I-E₂ for estrogen receptors. Mean tumor-to-blood ratios of 1.4, 5.5, 8.7 were seen at 1 h in rats with transplanted, spontaneous, and N-nitrosomethylurea-induced tumors, respectively. Better estrongen-receptor-binding radiopharmaceuticals can probably be designed