A series of parametric simulations was conducted to investigate the unsteady internal flow behavior in plain orifice, or "pressure" atomizers. The unsteady behavior is attributed to instabilities in the vena contracta and the presence of cavitation in this region. Even though the present calculations are laminar, they exhibit turbulent-like characteristics due to this instability. Orifice mass flow is shown to be periodic in most instances with fluctuations occurring at frequencies consistent with the orifice transit time. Cavitation is shown to enhance fluctuations, in some cases quite dramatically. Results are presented for a range of flow conditions and orifice geometries.