Allopurinol and oxypurinol influence the expression of key osteoblast genes. The effect of allopurinol and oxypurinol (0.1 μM) on the expression of collagen type I (Col1α1), osteocalcin (Ocn), TNAP, XO and ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) was in investigated in differentiating and mature osteoblasts. (A) Allopurinol decreased Npp1 expression ~5-fold in differentiating osteoblasts. (B) In mature osteoblasts, allopurinol increased TNAP and Ocn expression 2-fold and 5-fold, respectively. (C and D) Oxypurinol reduced Npp1 expression 5-fold in differentiating and mature osteoblasts. Levels of TNAP and Ocn expression were increased 2-fold and 5-fold, respectively in mature osteoblasts. Values are means±SEM (n=4), *=p<0.05.