The purified product called Vitamin D2 by Windaus was heated at 180-190[degree] for 4 hrs. From the products two substances of equal amounts could be separated[long dash]pyrocalciferol, and an isomeric alcohol, named iso-pyro-vitamin, the latter as the acetate, and the former as the dinitro-benzoate. The iso-pyro-vitamin was then reduced to the dihydro- and the hexahydro-compounds. The pyrocalciferol compound was similarly treated to form the dihydro- and hexahydro-derivatives and the corresponding epi-compounds. The melting point, optical rotation and elementary composition of these were obtained. Physiological tests, the results of which are included only in a summarized form, showed that doses of the heated vitamin as large as 2 mgm. were not toxic, in contrast to a 0.075 mgm. toxic dose of the unheated vitamin. Iso-pyro-vitamin gave negative results for antirachitic activity in a dose of 1 [gamma] and for toxicity 1 mgm. was negative. The pyrocalciferol showed no toxicity with 0.75 mgm. and no antirachitic value for 1 [gamma]. Irradiation with Mg light produced no activity in these substances.