During 1954, 1955, and 1956, 324 water samples were collected in western Kansas, eastern Colorado, the Oklahoma Panhandle, and northeastern New Mexico, to determine the uranium content of water from the various rock units and geologic terranes in the region, and to locate areas in which, large amounts of uranium in the water might reflect the presence of nearby uranium accumulations. Three geologic terranes are present in the report area: the Tertiary-Quaternary tuff ace o us terrane, the Upper Cretaceous shale terrane, and the upper Permian through Lower Cretaceous sandstone terrane. The average uranium content of 179 water samples from the tuffaceous terrane, which is composed primarily of tuffaceous, fluviatile rocks ranging in age from Pliocene to Pleistocene, is 6.7 parts per billion. A total of 48 samples from the Upper Cretaceous shale terrane, which is composed almost entirely of shale and limestone of marine origin, contain an average of 20.4 parts per billion uranium , Sandstone, silt stone, and clay stone of terrestrial and near-shore marine origin are the predominant constituents of the upper Permian through Lower Cretaceous sandstone terrane, and the 83 water samples collected from, or related to, rock units of this terrane contain an average of 10.2 parts per billion uranium. The average uranium content figure derived for the tipper Cretaceous shale terrane may not be representative of the uranium content of waters from this terrane throughout the report area because more than half of the samples assigned to the terrane were collected in an area in which most water samples contain relatively large amounts of uranium. Compared to the shale terrane, the tuffaceous terrane and the sandstone terrane are both represented by a greater number of samples collected over a much larger area and the average uranium content figures derived for them are believed to be representative of the uranium content of waters from, or related to, rock units of the two terranes in the report area. The average uranium content figures derived for different rock units, or groups of rock units, subdivided by the source type from which the samples were collected (well, spring, stream, municipal water system, reservoir, or lake), and in some cases by geographic parts of the report area, are believed to be of more potential use in any future hydrogeochemical exploration in the area than are the average uranium content figures derived for the three geologic terranes. The average uranium content of ground-water samples from 12 individual rock units or groups of rock units ranges from less than 1. 0 to 38 parts per billion. Several rock units were sampled over large enough areas to indicate that waters from the same rock unit in different parts of the report area may range widely in average uranium content. Water samples from some of the rock units in the area, particularly those of Triassic and Permian age, contain large amounts of uranium, and in some parts of the report area, such as the Cimarron River area of westernmost Oklahoma and northeastern New Mexico, and the Rule Creek area in Bent and Las Animas Counties, Colo. , most, or all, of the water samples collected contain relatively large amounts of uranium. Further exploration to determine the source of the uranium in the water from these rock units and areas may be worthwhile.