The insulating character of undoped manganites RMnO3 is related to the Jahn-Teller (JT) distortion of the MnO6 octahedra, which can be reduced by applying hydrostatic pressure. We analyze the dependence of the Raman phonons with pressure up to 10 GPa, for PrMnO3 and TbMnO3. The variation of the stretching and bending modes indicates that the compressibility of Mn-O bonds is higher than R-O ones in PrMnO3, but smaller in TbMnO3. The variation of the tilt mode frequency, in TbMnO3, can be explained by an increase of the octahedra tilt angle, which is consistent with a larger compressibility of the Tb-O bonds. Therefore, while PrMnO3 evolves towards the structure of the metallic-ferromagnetic doped perovskites, TbMnO3 does not. (C) 2003 Elsevier Science (USA). All rights reserved.