Because of its genetic effect, there has been much concern about gonadal exposure to man-made radiation. Geneticists (1) feel that there is no threshold dose below which mutations are not produced. Low doses are mutagenic, and all doses, however distributed, are cumulative. At the present time, diagnostic radiology is the major contributor to gonadal exposure of the populace. To the individual, however, exposure from radiotherapy is greater than that from any other source. The scope of the problem from the point of view of roentgen therapy is emphasized in the study made for the National Academy of Sciences by Laughlin and Pullman (2) who estimated that the number of x-ray treatments given by radiologists to persons under thirty in the year 1955 was 1.9 million. To this figure must be added the number of treatments given by dermatologists. Clark (3) estimated that 25 per cent of treatments in all age groups are for nonmalignant ailments. The remaining 75 per cent are for malignant diseases, where genetic effects are overlooked because of the severity of the affliction. Recent reports in the literature (4–10) indicate that comparatively little attention has been paid to the role of roentgen therapy as a contributor to gonadal exposure. It is true that the radiotherapist deals mainly with patients who have advanced malignant tumors, where the exposure received by the gonads is not of great significance. It is equally true, however, that he treats many benign conditions as well as certain malignant conditions, such as postoperative seminomas and early lymphomas in the young, where irradiation of the gonads may be of importance. The classic description of administration of radiotherapy invariably fails to mention protection of the gonads. The emphasis has rather been to shield normal tissues adjacent to the treatment field, radiotherapists having only recently become aware of the importance of gonadal shielding. It was with the foregoing considerations in mind that the study to be reported here was undertaken. Materials and Methods A series of measurements were made on male patients of varying habitus undergoing treatment for tumors in various sites. A 260-kv, self-rectified x-ray unit with a Regato field localizer was used for this investigation. Measurements of radiation around the gonads were made with a specially constructed cylindrical condenser type ionization chamber (Fig. 1) whose wall thickness was 1 mm. of Lucite with a collecting wire of 0.5 mm. A1. The volume of the chamber was 187 c.c., which is approximately the volume of the average adult male gonads. The chamber was used with a Victoreen electrometer, and the complete set was calibrated against a standard air-wall chamber. A “stepladder” Masonite pedestal in increments of 0.5 inch made possible height adjustment of the ionization chamber to the precise level of the gonads. The following constant factors were used: 250 kv, Thoraeus filter (h.v.1. 2.3 mm. Cu), 200 kv, 0.5 mm. Cu filter (h.v.l. 1.1 mm. Cu).