Numerous investigators have demonstrated that cod liver oil is the richest available natural source of fat-soluble vitamins. The antirachitic vitamin content of cod liver oil prepared and stored under proper conditions justifies its being considered as specific for rickets. More recently, investigators have demonstrated that ultraviolet light has definite therapeutic value in either the prophylactic or the curative treatment of patients with rickets. This is true of both the natural ultraviolet light present in sunshine and the ultraviolet light produced artificially by the mercury vapor quartz lamp and by some other sources of artificial light. As cod liver oil and ultraviolet light have been proved to be reliable agents for treatment in cases of rickets, the pediatrician naturally is interested to have information concerning the relative efficacy of cod liver oil and ultraviolet light as a source of antirachitic activity. This investigation was undertaken to obtain data which would shed