Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, 2-5-1 Akebono-cho, Kochi 780, Japan Biologically active glass (Bioglass) in the system Na20-CaO-SiO2-P2Os, is used as implant mater- ials because of the ability to bond chemically to living bone [1-3], but its mechanical properties, especially toughness, are insufficient for hard tissue. Therefore, their usefulness is limited to areas with- out mechanical requirements. Recent studies have demonstrated that dense ceramics can be toughened by rod-shaped particles dispersed in them, due to the crack deflection and pull-out toughening mechan- isms [4, 5]. Fully dense glass ceramics could be prepared by mechanical pressing under hydro- thermal conditions by the hydrothermal hot-pressing technique [6-10]. The present work deals with the preparation of reinforced Bioglass ceramics with rod-shaped crystals dispersion by hydrothermal hot- pressing. The autoclave for hydrothermal hot-pressing used in this work was a steel cylinder. Bioglass powders (45S5, Nikon Co., Japan) pulverized into grains under 200 mesh (smaller than 75 gm) were kneaded with 20 wt % distilled water, then transferred into the autoclave. The starting sample in the chamber was compressed uniaxially by cast rods from above and below. The details of this autoclave [11] and related autoclaves [12] were described in the previ- ous paper. The Bioglass ceramics were prepared by hydro- thermal hot-pressing at temperatures from 100 to 350 °C under 50 MPa mechanical pressing for 2 h. The produced phases were identified by powder X-ray diffractometry (XRD, Rigaku RAD-RC) with Ni-filtered CuKo~ radiation, operating at 40 kV and 100 mA. The microstructure of the glass ceramics was observed by scanning electron microscope (SEM, Hitachi $530) for the polished surface. According to the XRD analysis, the solidified Bioglass hot-pressed hydrothermally at 100 °C had no crystalline phases. This solidified Bioglass with- out crystalline phases had about 86% theoretical density. The Vickers hardness was determined to be 1.0 GPa by the micro-Vickers indentation method with 1.96 N loading for 15 s (Akashi MVK-EIII). The compressive strength of this glass was found to be 150 MPa by uniaxial compression perpendicular to the base of the compact in cylindrical shape with crosshead travel rate of 0.02 cmmin -1 (Shimadzu REH-100). XRD analysis (Fig. 1) revealed that the glass ceramics hot-pressed hydrothermally at above 200 °C had crystalline phases identified as NaCa2 0261-8028 © 1992 Chapman & Hall