In many types of nuclear-reactor-pumped lasers, the fission fragments that are used to excite gaseous lasing species heat the gas in a spatially nonuniform manner. This heating nonuniformity induces transient gas motion, which results in density and refractive-index gradients that affect the laser’s optical behavior. A computational model of the transient gas motion is developed using the acoustic filtering methodology, which neglects the spatial variation of the pressure. This model incorporates the effect of spatially varying gas density onfission-fragment heating. Gas motion out of the laser cell into small, rapidly cooled regions is treated as a volumetric mass loss distributed over the laser cell. Although these regions have a relatively small fraction of the total volume, a large amount of gas can flow into them during the heating because of the rapid cooling therein. This gas removal from the cell during pumping, neglected in previous analyses, is important because fission-fragment heating is strongly dependent on local gas density. To quantify the laser’s optical behavior, experiments are performed in which a probe laser beam is passed through the laser cell This probe beam acquires a wavefront distortion from the refractive-index gradients and is imaged onto a wavefront slope sensor, which yields temporally and spatially resolved measurements of the angular deflection (wavefront slope) of the probe laser beam. Experimental and computed results for this quantity exhibit reasonable agreement over a wide range of pressures and heating amplitudes.