The feasibility of a technique for autoradiography of diffusible substances has been further tested by analysing the localization of steroids in rat testes with the light-and electron-microscope. Testes of rats were perfused with tritiated testosterone (3 min) followed by 15-min perfusion with buffer containing a 100-fold of unlabelled testosterone. Tissue samples were frozen, freeze dried, fixed in osmium vapour and embedded in Epon. To exclude extraction of steroids, contact with water and other solvents was prevented during cutting of thin sections on an ultracryotome and further treatments for autoradiography. Light- and electron-microscopic observations indicate that the highest concentration of labelled testosterone was present within the basal parts of the Sertoli cell cytoplasm and in lipid inclusions of Sertoli cells within the seminiferous tubules. This is the first account of autoradiography of steroids at the electron-microscope level.

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