and the Dajac Laboratories of the Chemical Division of the Borden Company.Acknowledgement is due Mrs. Soveig Varnauskas for technical assistance, and Mr Harold Thomas for photomicrography. Research Fellow of the Rockefeller Institute for Medical Research. * One exception to this observation was noted. Apparently the high concentration of lipid within the adrenal cortex and its characteristic steroid composition favors the formation of crystal clumps. This phenomenon was not observed in tissue sections of the central nervous system. 422 MARVIN M. NACHLAS ET AL. bation solution was needed, 10 ml. of the buffered substrate reagent were added to 10 ml. of an aqueous solution containing 10 mg. of Nitro BT.* During this study, three additional solutions were prepared to compare the activating effect of certain ions upon succinic dehydrogenase activity. These were calcium chloride (0.03 M), * This reagent may be purchased from the Dajac Laboratories, Chemical Divuion,