The vitamin D-3-dependent calcium binding protein, calbindin-D28K (CaBP-D28K), plays an important role in transepithelial calcium translocation. To evaluate its role in chick embryonic calcium metabolism, steady-state levels of CaBP-D28K mRNA in various tissues of the chick embryo were determined by Northern blot and slot blot analyses, and CaBP-D28K concentrations in the examined tissues and plasma were estimated by RIA. High levels of CaBP-D28K mRNA were found in the embryonic kidney (mesonephros) on embryonic day (E) 10 and E12 and thereafter gradually decreased until hatching. CaBP-D28K mRNA levels were low in the yolk sac until E16 but increased on E18 and reached a maximum on E20. A steady increase in CaBP-D28K mRNA levels was observed in the cerebellum during the development from E10 to post-hatching. CaBP-D28K mRNA levels in the intestine were very low during the incubation period but significantly increased on days 1 and 7 after hatching. By Northern blot analysis, CaBP-D28K mRNA was barely detectable in liver, heart, and chorioallantoic membrane of the embryonic chick. Changes in immunoreactive CaBP-D28K of each tissue paralleled observed changes in mRNA levels. In plasma, measurable levels of CaBP-D28K were found as early as E8 and were stable until E18, when 6.5-fold increase was observed compared to E16. The highest level of CaBP-D28K in plasma was found on E20 and decreased after hatching. These temporal profiles of CaBP-D28K suggest that it may play an important role in the regulation of chick embryonic calcium homeostasis. (C) 1994 Wiley-Liss, Inc.