The relationship between structure and pressure for MgSiN2 has been simulated using a d. functional approxn. within the local d. approxn. The low-pressure (LP-) phase of MgSiN2 with an orthorhombic structure transforms to a high-pressure (HP-) modification with a CsICl2-type structure at a pressure of about 16.5 GPa. HP-MgSiN2 , in which both Mg and Si are octahedrally coordinated by N, has a bulk modulus of about 238 GPa, much higher than that of the LP-modification (about 182 GPa) with tetrahedrally coordinated metal atoms. HP-MgSiN2 is a wide-gap semiconductor with an indirect energy gap of about 4.3 eV, similar to that of LP-MgSiN2. The direct gap at G is about 5.8 eV.