A new procedure is described for silver impregnation of thin sections for electron microscopy. Sections of various tissues, fixed in OsO 4 and embedded in methacrylate, were treated with an ammoniacal silver solution, directly or after oxidation with periodic acid or hydrogen peroxide. After OsO 4 fixation all cellular membranous systems exhibit a primary argentaffinity probably due to the reduction of ammoniacal silver solution by the reduced osmium bound to unsaturated lipids. Bleaching the sections with hydrogen peroxide removes the argentaffinity of protoplasmic structures. Treatment of the sections with periodic acid results in decreased argentaffinity of protoplasmic components while the argentaffinity of metaplasmic structures is greatly enhanced. The latter procedure appears particularly useful for enhancing the contrast of basement membranes.