New shock wave data on {100} oriented single crystal periclase covering the pressure range from 160 to 200 GPa suggest that MgO is described by a single Hugoniot up to 200 GPa, with no displacive phase transitions of volume change greater than 1--1.5 per cent. For a third order finite strain fit, with K0 constrained to its ultrasonically determined value of 162.7 GPa, the implied K0' of 4.27¿0.24 is in agreement with ultrasonically determined value of 4.17¿0.14. The new data indicate a somewhat steeper Hugoniot than that suggested by previously published shock wave results under 120 GPa. A previously published result at 258 GPa shows more compression in the light of the present data than would be expected for MgO in the B1 structure, and may signal the onset of a phase transition, although we cannot confidently make this interpretation. If MgO forms an ideal solid solution with FeO, our data does not support the occurrence of a significant transition in magnesiowustite at lower mantle pressures.