The aim of this study was to investigate whether 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), the active metabolite of vitamin D, regulates the expression of rat placental calbindin-D9K mRNA. One group of rats (-D) was fed a vitamin D-deficient diet before and during pregnancy, whereas a second group (+D) was fed a vitamin D-replete diet over the same period. Animals were killed on d 21 of gestation, and plasma concentrations of 1,25(OH)2D3 for +D and -D animals were significantly (p < 0.05) different (260 +/- 78 and 39 +/- 8 pM in maternal plasma and 122 +/- 39 and 42 +/- 10 pM in fetal plasma, respectively; mean +/- SE, n = 4-5). Vitamin D deficiency had no effect on placental weight, fetal weight, fetal ashed weight, fetal calcium accretion, or the maternofetal calcium gradient. Hybridization of RNA from maternal duodena (used as a positive control tissue) and placentas of +D and -D rats with a rat calbindin-D9K cDNA revealed a single 0.6-kb transcript in both tissues. The abundance of this transcript was markedly lower (p = 0.06) in the duodena of -D compared with +D rats (mean change -68 +/- 9%) but there was no difference between the placentas of the two groups (mean change +13 +/- 22%). These changes were significantly (p < 0.05) different between the two tissues and the response of each tissue to vitamin D deficiency was significantly different (p < 0.01). These data indicate that 1,25(OH)2D3 does not regulate the expression of calbindin-D9K mRNA in rat placenta.