Using tritiated oestradiol of very high specific activity and sensitive counting techniques, Jensen & Jacobson (1962) were able to investigate the distribution of this hormone among target and other organs after the injection of physiological doses into Sprague—Dawley rats. It is also possible to demonstrate the uptake of labelled oestradiol and its distribution within the tissues by the conventional autoradiographic stripping film technique after incubation of the tissues in very low concentrations of the hormone. Virgin female Sprague—Dawley rats, approximately 12 weeks old of a mean weight of 160 g., were used. They were killed by dislocation of the neck, and small cylinders (2–3 mm. in length) of the uterine horns and of small intestine, and also small pieces of liver (maximum thickness 3 mm.), were placed in tissue culture medium '199' containing (6,7-3H)oestradiol-17β (specific activity 150 mc/mg.; New England Nuclear Corporation). The hormone was added