Tension tests to fracture under superposed hydrostatic pressure reaching to nearly 30 000 kg/cm2have been conducted on five polycrystalline metals, Ni, Ta, Cb, Mo, and W, two brittle single metallic crystals, Sb and Cu5Zn8, and two brittle amorphous materials, ``Melmac 404'' and B2O3glass. All these materials exhibit ductility at the highest pressure; the largest increases are for the polycrystalline metals of which the largest is W, which breaks brittely at atmospheric pressure but with 81 percent reduction of area at the highest pressure. The shape of the strain hardening curves varies among the five metals from abrupt rise with long tailing off to long gradual rise with comparatively abrupt falling off, in the order W, Mo, Ta, Cb, and Ni. The character of the fracture varies continuously with pressure and is different for the different metals. Of the brittle single crystals Sb has a reduction of area of 55 percent at the highest pressure. The plastic, Melmac 404, gives the smallest increase of ductility, a reduction of area of only 14 percent at the maximum. This small value is doubtless associated with the absence of internal crystal slip planes. B2O3glass gave a reduction of 87 percent; this is largely a phenomenon of plasticity, but there is an appreciable component of viscosity. Both amorphous materials showed an increase of density after pulling under pressure instead of a decrease.