Low Froude number ventilated cavities developed behind a disk cavitator, in a horizontal flow, under the influence of a vertical gravity field are considered. Pulsation is shown to occur in two different regions on the C_Q − σ phase map, the hysteresis region and at vent rates greater than those necessary to maintain twin vortex cavities. Two conditions are necessary to initiate pulsation. One condition is a ventilation rate too great for either re-entrant jet or twin vortex closure regimes. The other condition is a finite amplitude pressure wave needs to force the cavity into pulsation.