Objective:To determine effects of parathyroid hormone (PTH) and different dietary calcium and phosphate on synthesis of active vitamin D. Methods:Two-month-old wild-type(WT) and PTH null (PTH-/-) mice had been fed on a normal diet (1% calcium,0.64% phosphate),high calcium low phosphate diet(2% calcium,0.4% phosphate) or low calcium low phosphate diet (0.005-0.01% calcium,0.4% phosphate) for 4 weeks,and serum calcium,phospharus,PTH and 1,25(OH)2D3 levels were measured. The expression of renal 25-hydroxyvitamin D 1α-hydroxylase[1α(OH)ase]gene was assessed using RT-PCR. Results:Serum calcium levels were decreased,serum phospharus levels were raised. The expression of renal 1α(OH)ase gene was up-regulated,whereas serum 1,25(OH)2D3 levels were reduced in PTH-/- mice on the normal diet compared with their WT littlemates on the same diet. Serum PTH and 1,25(OH)2D3 levels were reduced and the expression of renal 1α(OH)ase gene was down-regulated in WT and PTH-/- mice on the high calcium low phosphate diet compared with genotype-matched mice on the normal diet. Serum calcium and phospharus levels were normal,and the expression of renal 1α(OH)ase gene was still up-regulated with same level of 1,25(OH)2D3 in PTH-/- mice on the high calcium low phosphate diet compared with their WT littlemates on the same diet. Serum PTH and 1,25(OH)2D3 levels were raised significantly and the expression of renal 1α(OH)ase gene was up-regulated dramatically in WT and PTH-/- mice on the low calcium low phosphate diet compared with genotype-matched mice on the normal diet. Serum calcium levels were reduced more dramatically,and serum phospharus levels were less higher,with expression of renal 1α(OH)ase gene and lower serum 1,25(OH)2D3 levels in PTH-/- mice on the low calcium low phosphate diet compared with their WT littlemates on the same diet. Conclusion:Synthesis of active vitamin D was regulated by dietary calcium and phosphate in both PTH-dependent and PTH-independent manners.