Measurements were made of duodenal calcium-binding protein (CaBP) on rats during development of rickets and, subsequently, following vitamin-D2 treatment.Results showed a poor inverse correlation between duodenal CaBP and rickets. In rats fed a phosphate-deficient rachitogenic diet, duodenal CaBP concentration finally fell below detectable limits, but CaBP was still readily measurable 2 weeks after rickets was clearly established. Following a massive dose of vitamin D2 (50000 I.U.) to rachitic animals, CaBP was formed. However, a small dose of vitamin D2 (500 I.U. daily for 4 days) was insufficient to demonstrate CaBP synthesis even though it was sufficient to demonstrate calcification of cartilage. Administration of normal rat chow to rachitic rats stimulated greater CaBP synthesis than vitamin-D treatment alone. The rachitogenic diet supplemented with phosphate, which caused osteoporosis but not rickets, inhibited CaBP synthesis. The results suggest that nutritional deficiencies from the rachitogenic diet, in addition to vitamin-D deficiency, inhibited CaBP synthesis.