Through a global three-dimensional MHD simulation, which makes it possible to reveal the physical processes and causalities of the global interaction between the solar wind and the magnetosphere (Watanabe and Sato, 1990; Kageyama et al., 1992), the effect of the interplanetary magnetic field (IMF), both northward and southward, on Earth's magnetosphere has been investigated. A southward IMF, upon reconnecting at the dayside magnetopause, sweeps the Earth's magnetic field toward the nightside and drapes the magnetotail. This gives rise to a plasma sheet cross-tail current increase and explosive magnetic reconnection at around 15RE from the Earth. This reconnection results in the formation of a large plasmoid which grows much faster than for the simulation with no IMF, thus supporting previous notions that a southward IMF is a driving mechanism of plasma sheet reconnection. A northward IMF is observed to reconnect with the magnetosphere along the cusp shoulder, stripping magnetic field lines away and weakening compression of the plasma sheet, thus inhibiting plasma sheet reconnection. In order to accommodate a balance of magnetic to dynamic pressure along the magnetopause, the magnetosphere changes from its usual cometlike shape to that resembling a tadpole as it attempts to return to a dipolar structure. Plasma sheet reconnection is thereby inhibited. A case when the direction of the IMF turns from south to north is also studied. ¿ American Geophysical Union 1993