Heat release in a laser with subsonic or supersonic flow of the lasing medium causes variation in gas density. Variations in gas density result in phase differences at the exit aperture of the laser beam. Beam quality is degraded. Using the linearized solutions for a line source of heat in subsonic, planar flow, equations are derived for the density perturbation Lip p. Lasers having subsonic flow in the cavity include electroaerodynamic lasers. The equations permit assessment of Mach number effects and wall shape. As Mach number increases, the influence of the heat source wakes becomes less important. A wall shape equivalent to operation in a flow of infinite extent gives smaller Lp/p than par-allel walls. To apply the aforementioned equations, it is necessary to know the heat release function, h(x,y). A simplified vibra-tional relaxation model is developed and coupled with Joule heating to arrive at estimates for h(x,y). The model predicts populations with and without laser power extraction.