1. The short- and long-term effects of oral phosphate supplements were studied in two groups of adult rabbits. 2. In the short-term study, compared with control animals, the bones of phosphate-supplemented rabbits showed a greater degree of porosity, and defects in the tibia created at the beginning of the study were replaced with bone tissue containing many areas of resorption. There was an increase in retention of 85Sr in the hard and soft tissues, particularly in the kidney and thoracic aorta. 3. In the long-term study, a second group of rabbits received a low calcium and phosphate intake during the control period; half of this group were then fed a low dosage of supplemental phosphate while the remaining half remained on the control diet. After a period of 6 months the bones of phosphate-supplemented animals showed increased porosity and there was an increase in 85Sr retention by the kidney compared with the control group. 4. Phosphate supplementation appears to result in secondary hyperparathyroidism and to cause soft-tissue calcification.