INTRODUCTION For the past several years, HASL has maintained a continuing interest in the natural radiation environment, with particular emphasis on: its more penetrating components.’ Sensitive ionization chambers have been developed to measure the total dose rate from both terrestrial and cosmic radiation sources.* The presence of significant quantities of gamma-emitting fission products in the environment as a result of nuclear weapons test- ing necessitated the development of techniques for inferring the contribution of fallout to the total measured dose rate. Relatively simple methods of analysis have been developed which allow reasonably precise estimates of the individual components of the terrestrial gamma radiation field to be made from a pulse height spectrum obtained in the field with a 5" x 3" NaI(T1l) detector. This report summarizes these methods® and presents the total terrestrial and individual component dose rates determined from spectrometric and ionization chamber measurements taken in 1962 and 1963 at numerous locations in the southeastern, central and western United States. Most of these locations were grouped in a few areas of particular interest (e.g., Denver and Colorado Springs, Colo.; the San Francisco Bay area; the Olympic Peninsula, Washington; and a region near Aiken, S.C.) for studying the nat- ural radiation environment and for verifying our methods of determining dose rates. A number of spot measurements were also taken enroute to these areas from our laboratory in New York City. SURVEY TECHNIQUES Choice of Site Survey locations were usually city parks, school or church lawns, or vacant lots providing fairly large (at least 30 ft. in diameter), grassy, flat areas, whose soil appeared to be fairly typical of the locale. The requirement for flatness is important for two reasons: first, the interpretation of the pulse height spectra in terms of dose rate relies on the assumption of relatively uniform distribution of emitters ina flat "half space"; second, ground depressions often show elevated fallout gamma dose rate levels.