Experiments of VF were made on 77 mongrel dogs under hypothermy. In 24 donors VF was produced through mechanical trauma. Then, an aqueous extract was prepared from the fibrillating ventricles and this was injected into the coronary arteries of the recipients. Based on the above data, the author concludes that: (1) the aqueous extract contains the factor(s) which is (are) capable of producing VF; (2) the principal cause of VF is not the potassium or calcium content found within the extract; (3) the factor(s) which is (are) responsible for the production of VF is (are) not proteic.