It has long been known that the ovary is particularly sensitive to radiation, and that it also shows the effect of selective action in a marked degree. For these reasons the ovary of the rabbit has been one of the tissues chosen for studying the differential action of gamma radiation. As frequent use will be made of the two terms “differential” and “selective” action, it may be as well to distinguish them.Selective action:When a beam of radiation passes through a collection of cells some varieties are more affected by the irradiation than others, quite irrespective of the variation in the intensity of the irradiation as it goes from one point to another or from plane to plane. The ovary is a good example. For years it has been known that the ovary can be put temporarily, or even permanently, out of action with a dose of X rays which will cause no appreciable damage either to the skin or to the tissues lying between the skin and the ovary. Moreover, the ovary further shows selective action in that the cells of which it is composed vary enormously in their sensitiveness to radiation; with a particular dose it is possible practically to destroy the ova and Graafian follicles without doing any appreciable damage to the rest of the ovary.Differential action:Differential action is said to occur when different biological effects take place in any particular tissue after exposure to radiation of different wave-lengths, although the actual amount of energy liberated in the tissues is the same.The experiments detailed in this paper have been made to see whether any differential action is shown in the effects of radiation on the ovary of the rabbit when using different groups of gamma rays from radium. Before any biological work could be done it was necessary to make very careful physical measurements of the tubes used, so that the total effect of the radiation and not only that of the harder gamma rays might be taken into account. The technical details of the radium tubes are given in Table I.