The influence of the vitamin D status on the concentration of the serum vitamin D-binding protein was studied. In normal rats the serum vitamin D-binding protein increases gradually from birth to adulthood; after puberty a higher concentration is found in male rats than in female rats. Rats fed a vitamin D-deficient diet containing a sufficient amount of calcium and phosphorus were found to have a normal DBP (binding protein for vitamin D and its hydroxylated metabolites) pattern indistinguishable from that of rats receiving the same diet but supplemented with a weekly injection of 500 microgram vitamin D3. Rats fed a vitamin D-deficient, low calcium, low phosphorus diet developed severe hypocalcemia and growth retardation, but their DBP pattern was not significantly different from that of rats fed the same diet but supplemented with a weekly injection of 500 microgram vitamin D3. The concentration of the transport protein for vitamin D was thus unrelated to the vitamin D status of the rat.