The motion of plasma density enhancements (barium clouds) artificially introduced into the postsunset equatorial F region is investigated with a two-dimensional model incorporating flux tube integrated quantities. The temporal development of the ionosphere, in which the density perturbations are imbedded, is derived from a one-dimensional set of relations modeling plasma transport and the vertical electric field from initial conditions and a prescribed variation of the horizontal electric field as a function of time. The calculations show that the strong horizontal shear flows existing at the nighttime F region ledge (where the perturbations were placed) reduce the growth of polarization fields associated with the enhancements and adjacent relative depletions of plasma for weak perturbations. The reason is that the perturbations develop a strong tilt with respect to the horizontal. More massive density perturbations lead to stronger drop velocities with respect to the rising ambient plasma. At a later time they develop secondary horizontal density perturbations on the side unstable to E × B drift instability due to the motion of neutral constituents. When rising “bubbles” of low density are produced, they (1) form on the steepened eastward side of the enhancement perturbation, (2) have a width comparable to the scale size of the enhancement perturbation, and (3) are most easily produced when the enhancement perturbation size is comparable to the scale height of the integrated density. These simulations show why experimental efforts of initiating rising bubbles and equatorial spread F have not been successful. The experiments require larger-scale and stronger density perturbations than what can be achieved with conventional sounding rocket releases of barium vapors.