['The study is designed to determine whether estrogen and vitamin D endocrine systems interact to regulate calcium (Ca) balance as well as changes in mRNA expression of epithelial Ca transport proteins involved in intestinal and renal Ca transport in aging animals in response to ovariectomy and low dietary Ca intake.', 'Eleven-month-old female sham or ovariectomized (OVX) rats were divided into four groups and fed with either a low-Ca (LCD; 0.1% Ca, 0.65% P) or a high-Ca (HCD; 1.2% Ca, 0.65% P) diet for 12weeks. Ca balance and mRNA expression of Ca transport proteins in the intestine and kidney from rats were systematically studied.', 'OVX rats fed with LCD resulted in a negative Ca balance. LCD suppressed serum Ca in OVX but not sham rats, resulting in an induction of serum PTH and 1,25(OH)2D3 levels. The surge in serum 1,25(OH)2D3 levels in LCD-fed OVX rats was associated with an increase in mRNA expression of intestinal transient receptor potential cation channel (TRPV6) and calbindin D9k (CaBP9k) as well as renal vitamin D receptor (VDR), but such an induction was unable to restore Ca balance in vivo. In contrast, the negative Ca balance was associated with suppression of intestinal plasma membrane Ca pump (PMCA1b) and renal transient receptor potential cation channel (TRPV5), calbindin D28k (CaBP28k) and PMCA1b mRNA expression in aged OVX rats.'] Is estrogen deficiency-induced Ca balance impairment associated with decrease in expression of epithelial Ca transport proteins in aged female rats?