Small-angle neutron scattering experiments were performed on horse azidometmyoglobin (MbN3) at pressures up to 300 spectroscopic techniques have shown that a reorganization of the secondary structure and of the active site occur in this pressure range. The present measurements, performedusingvarious concentrations ofMbN3, showthat the compactness of the protein is not altered as the value of its radius of gyration remains constant up to 300 MPa.