A technique has been described for preparing radioautographs of germ cells in intact segments of seminiferous tubules from rat testis. Pieces of tubules were dissected from testes, fixed in Bouin's fluid and stained in Harris hematoxylin. For radioautography, rectangular coverslips were first dipped in liquid Kodak NTB2 emulsion. The tubules were placed on the partially set emulsion, and thus became evenly coated on their viewing surface with a thin layer of emulsion. Following routine exposure and processing, the coverslips bearing these tubules were inverted onto microscope slides to give the final whole mount radioautographic preparation.