The influence of a high phosphorus (1.5%) and high calcium (2.2%) diet on ectopic mineralization in boars was examined over a four month period. The high phosphorus diet caused metastatic mineralization in the left atrial endocardium in 84% of animals and in the pulmonary and diaphragmatic pleura in 21 and 58% of animals, respectively. Mineralization, that apparently commenced as deposits of extraneous calcium and progressed by metastatic deposition, occurred also in the lamina propria mucosae of the respiratory airways and in the lamina muscularis mucosae of fundic stomach. Hyperplasia of osteoclasts and microfractures in costochondral junction, morphological features of activated parathyroid cells and a significant drop in serum phosphorus during the fourth month in boars fed high phosphorus, suggest that nutritional hyperparathyroidism was experimentally induced. No systemic bone disease developed. Feeding a high calcium diet resulted in 20% incidence of discrete lesions in the left atrial endocardium and no pleural involvement. Also, lesions in respiratory airways and fundic stomach were mild. The fact that high phosphorus but not high calcium increased the incidence and extent of ectopic mineralization suggests that hyperparathyroidism under normocalcemic conditions might be involved in the pathogenesis of these lesions.