SUMSixty-four samples from various sites along the granite batholith of SW. England, from the surrounding country rock, and from Lundy Island have been analyzed for the radiogenic elements potassium, uranium and thorium. An overall picture emerges of a radioactive batholith, uniform along its length, emplaced in a normal and less radioactive country rock. Weathered and kaolinized granites are depleted in U and Th but not K, relative to fresh granite. A proportion of the uranium mineralization found in the region, as well as the many different ages determined for uraninite and coffinite mineralization, are probably caused by the remobilization of the MARY uranium which has been leached from the granites. A traverse across the Dartmoor pluton revealed no marked differences between the megacrystic and poorly megacrystic granites. In contrast, the radiogenic concentrations of the fine grained granites which comprise parts of the Land's End and Carnmenellis plutons are atypical and are possibly rafts of assimilated country rock. Assuming that the radiogenic elements are distributed in exponentially decreasing amounts with depth, then the measured data are consistent with the formation of the batholith near the bast. of an originally normal crust with a subsequent high-level emplacement.THE PURPOSE OF THE STUDY was to utilize the radioactive, heat-producing elements K, U and Th to investigate the uniformity of the various plutons and to study the differences with the country rock. It was hoped that the investigation would assist in the understanding of the geology and petrogenesis of the granite as well as in explaining the uranium mineralization of the region. Radiogeology generally consists of detailed radiometric mapping with scintillation techniques, usually by air or car. Comprehensive radiometric investigations of this type have been made in SW. England, and throughout much of Great Britain, by the Institute of Geological Sciences (Bowie et al. 1973, I.G.S. Annual Reports 195i-7 2) but remain virtually unpublished. The type of survey reported here makes amends for its discrete, less complete areal coverage by yielding absolute, as opposed to relative, elemental concentrations; it is the first of its kind in the British Isles.