On the basis of resistive MHD simulations we investigate the stability and dynamic evolution of the magnetotail for configurations with various degrees of plasma sheet thickening from midnight toward the tail flanks, associated with a similar increase of the Bz component. This increase is varied by factors between 2 and 3.5, while the magnitude of Bz at midnight is left unchanged. The increase of Bz has a strong effect on the spatial extent of resistive tearing and reconnection across the tail and on the unstable evolution. For stronger thickening the cross-tail extent of the near-Earth neutral line, formed by reconnection, and correspondingly the amount of reconnected magnetic flux get reduced and the dipolarization effects earthward of the neutral line are more concentrated near midnight. An increase of Bz from midnight toward the flanks by more than a factor of about 3 can possibly even suppress the resistive tearing instability. This indicates the possibility of a stability transition due to a reduction of Bz in the flank regions in response to changes in the solar wind, even when the magnitude of Bz near midnight and the fluctuation level in that region are unchanged (provided that a sufficient fluctuation level exists). ¿ American Geophysical Union 1994