Bone density and composition were studied in trabecular and cortical bone from control 2 and 4 year old beagles and those which had various degrees of renal failure as a result of perinatal irradiation. Changes in the two types of bone were qualitatively similar but consistently greater in trabecular bone. In a group identified as markedly uremic, the decrease (P less than 0.025) from control levels in specific gravity was about 4 times greater in trabecular than in cortical samples. The decrease (P less than 0.025) in grams of ash/ml was 9% in trabecular bone and 2% in cortical bone. These changes were associated with an increase (P less than 0.001) in water content and, on a percent by volume basis, approximately equal decreases in ash (P less than 0.025) and combustible matter (P less than 0.025). In a mildly uremic group there were similar trends in mean values but the only significant difference was an increase (P less than 0.001) in trabecular bone water.