The striking effects of small doses of irradiated ergosterol in the prevention and treatment of rickets have been shown by Hottinger,1Rosenheim and Webster,2Hess, Weinstock and Sherman,3Hess and Windhaus,4Kreitmair and Moll5and A. F. Hess.6 Heretofore it has been impossible to study the effects of large doses of vitamin D because of impurities associated with the vital factor. The development of irradiated ergosterol has made it possible to administer tremendous doses to see what pathologic conditions, if any, would occur. Albino rats weighing between 50 and 70 Gm. were fed on Steenbock7diet number 2965 (a low phosphorus ricketogenic diet) for four weeks. At the end of this time they showed definite rickets on section of the tibias and on roentgen examination. The animals were then divided into three groups. One group, which served as controls, was exposed to reflected