Concentrations and organ distribution patterns of alpha-emitting isotopes of U (238U and 234U), Th (232Th, 230Th, and 228Th), and Pu (239,240Pu) were determined for beagle dogs of our colony. The dogs were exposed to environmental levels of U and Th isotopes through ingestion (food and water) and inhalation to stimulate environmental exposures of the general human population. The organ distribution patterns of these radionuclides in beagles are compared to patterns in humans to determine if it is appropriate to extrapolate organ content data from beagles to humans. The results indicated that approximately 80% of the U and Th accumulated in bone in both species. The organ content percentages of these radionuclides in soft tissues such as liver, kidney, etc. of both species were comparable. The human lung contained higher percentages of U and Th than the beagle lung, perhaps because the longer life span of humans resulted in a longer exposure time. If the U and Th content of dog lung is normalized to an exposure time of 58 y and 63 y, median ages of the U and Th study populations, respectively, the lung content for both species is comparable. The organ content of 239,240Pu in humans and beagles differed slightly. In the beagle, the liver contained more than 60%, and the skeleton contained less than 40% of the Pu body content. In humans, the liver contained approximately 37%, and the skeleton contained approximately 58% of the body content. This difference may have been due to differences in the mode of intake of Pu in each species or to differences in the chemical form of Pu. In general, the results suggest that the beagle may be an appropriate experimental animal from which to extrapolate data to humans with reference to the percentage of U, Th, and Pu found in the organs.