I n vitro 1,25-dihydroxycholecalciferol (1,25(OH)2D3) decreased levels of preproparathyroid(preproPTH) hormone mRNA. We have now pursued these studies in vivo in the rat. Rats were administered vitamin D metabolites i.p. and the levels of preproPTH mRNA were determined in excised parathyroid-thyroid glands by blot hybridization. PreproPTH mRNA levels were less than 4% of basal at 48 h after 100 pmol 1,25(OH)2D3, with no increase in serum calcium. Gel blots showed that 1,25(OH)2D3 decreased preproPTH mRNA levels without any change in its size (833 basepair). Microdissected parathyroids after 1,25(OH)2D3 (100 pmol) showed mRNA levels for preproPTH were 40 +/- 8% of controls, but for beta-actin were 100% of controls. The relative potencies of vitamin D metabolites were: 1,25(OH)2D3 greater than 24,25(OH)2D3 greater than 25(OH)D3 greater than vitamin D3. In vitro nuclear transcription showed that 1,25(OH)2D3-treated (100 pmol) rats' PTH transcription was 10% of control, while beta-actin was 100%. These results show that 1,25(OH)2D3 regulates PTH gene transcription. PTH stimulates 1,25(OH)2D3 synthesis, which then inhibits PTH synthesis, thus completing an endocrinological feedback loop.