The fetal uterus shows a significant retention of the radioactivity with values that are 3.5-12 times higher than in the other fetal organs. Furthermore, the uterus, lung, kidney, and intestine show an increase in the uptake of radioactivity with time, with maximal values 0.5-1 h after the injection; the heart, brain, adrenal, and liver show their greatest uptake at 10 min after the injection, with decreasing values thereafter. The analysis of the radioactive material shows that unmetabolized [³ H] -estradiol is largely predominant in the uterus 6 h after administration of [³ H] -estradiol to the fetus (94% of the total radioactivity in this organ), although a relatively significant amount of [³ H] -estrone is found, especially in the cytosol fraction, at the early intervals after [³ H] -estradiol administration. Important quantities of [³ H] -estrone are also present in the lung (12-45%) and brain (27-65%). In the liver and intestine, the greater part of the radioactive material is metabolic products of [³ H] -estradiol (estrone, and estroneand estradiol sulfates). Most of the radioactive material in the plasma is estrogen sulfates (53-69%), particularly estrone sulfate. An intense sulfokinase activity is observed in the fetal adrenal and liver. In the fetal uterus 10 min after injection of [³ H] -estradiol, most of the radioactivity is localized in the cytosol fraction (59% of the total radioactivity of the organ), but at 6 h the nuclear radioactivity represents 68%. This nuclear radioactivity is mainly unmetabolized [³ H] -estradiol (97% of the total radioactivity of this fraction) which remains at a high concentration 6 h after administration. On the other hand, the concentration of [³ H] -estrone in this tissue diminishes very rapidly.

This selective uptake and long retention of [³ H] -estradiol in the fetal uterus of guinea pig can be related to the presence of the large quantity of estradiol receptors found in this fetal organ in previous studies in this laboratory.