Serious damages have been reported in hydraulic structures due to cavitation.  It is known that introducing air into the flow will considerably reduce cavitation risks.  Thus, aerators are used in many situations to introduce air into the flow and to eliminate cavitation tendency.  Although many works have been reported for flow aeration in outlet conduits of rectangular sections, less information can be found for circular tunnels.  These structures are commonly used to supply water for hydroelectric power plants or drinking and irrigation systems. In this paper, experimental information regarding the intensity of pressure fluctuations and its dominant frequency downstream of rectangular aerators in such structures are provided. Experiments were performed in a circular pipe of 28.8 cm diameter. Rectangular aerators of different heights and angles were fixed inside the tunnel and air was supplied through aerator to check its effect on the intensity of hydrodynamic pressures.  This information is useful for the structural design of such outlet tunnels.