Consolidated pherytoin and indomethacin by hot pressing were irradiated by Nd:YAG laser either at 266 or 1064 nm, to produce nanoparticles of the drugs. The average primary particle size recognized by transmission electron microscope (TEM) was below 20 nm, while the amount of decomposed fraction was well below the detection limit of H-1-NMR and high performance liquid chromatography (HPLC). Laser wavelength and power density were controlled to selectively break the intermolecular van der Waals force while avoiding molecular decomposition. Mechanism of reagglomeration of the ablated clusters without decomposition was elucidated in terms of the transformation of electronic to vibrational excitation during ablation. (C) 2004 Elsevier B.V. All rights reserved.