1. In all age groups the effects of ammonium chloride administration were found to be strikingly dependent upon the calcium intake.
2. Dogs receiving an adequate calcium diet and ammonium chloride showed less decalcification than those receiving a low calcium diet with or without ammonium chloride.
3. In the younger groups the added effect of ammonium chloride to calcium-low diet brought out more striking changes than a low calcium diet alone.
4. When the decalcification was less severe—in the oldest dogs on the low calcium diet with or without ammonium chloride, and in the younger dogs on an adequate calcium intake with ammonium chloride —generalized thinning of the bones without marrow fibrosis resulted (osteoporosis).
5. When the decalcification was rapid and severe—in the youngest dogs on low calcium diet, particularly with ammonium chloride— generalized decalcification and secondary marrow fibrosis resulted (ostitis fibrosa).
6. Generalized ostitis fibrosa is a rather inclusive term and may be applied to the histologic picture which results when clinical or experimental decalcification is rapid, and therefore leads to extensive marrow fibrosis.
7. The special underlying causes of the decalcification may incidentally contribute features to modify the generalized osteoporosis or ostitis fibrosa as in rickets and in von Recklinghausen's disease.