Hydroxyapatite (HAP) rods were synthesized from dicalcium phosphate anhydrous (CaHPO4, DCPA) and calcium carbonate (CaCO3) by the hydrothermal method from 120 to 180 degrees C. Both cuttlebone (aragonite polymorph of CaCO3) and CaCO3 chemical (calcite polymorph of CaCO3) were used as CaCO3 sources. The nucleation and growth of HAP rods mainly occurred on DCPA particles, while some HAP rods also grew from aragonite particles. The nucleation and growth of P-tricalcium phosphate (beta-TCP) particles on the surface of calcite particles were observed at the beginning of the reaction of DCPA and calcite, and some HAP rods were also found to grow out of beta-TCP particles. After the hydrothermal reaction at 140 degrees C for 24 h, most products are HAP with a small arnount of beta-TCP synthesized as a byproduct. The HAP rods synthesized were similar to 200nm in width and several microns in length. The reaction mechanism and growth process of HAP rods are discussed. (C) 2007 Elsevier B.V. All rights reserved.