Little information is available on the uptake of progesterone by various tissues of the rat. In the present investigation ovariectomized adult female rats were primed with oestradiol and the disappearance of [1,2-3H]progesterone was studied after a single injection. Tissue and blood samples were removed after varying intervals of time and radio activity was determined with a liquid scintillation counter (Packard, Model 3324) after dissolving the tissues in hyamine. Tritiated toluene was used as an internal standard for quenching corrections. The results are presented in Fig. 1. The data indicate that progesterone (and its metabolites) is rapidly taken up by the brain and pituitary. The portion of brain taken for determination of radioactivity included a part of the hypothalamus and adjoining cortical areas. Within 2 min. of the injection, maximum radioactivity, which is of the order of 7·2 × 10−9 and 5·7 × 10−9 moles/kg., respectively, was