Abstract: A ventilation system for a livestock building to provide a controlled environment therein. The ventilation system includes one or more exhaust fans for withdrawing air from the building, and also includes a controllable baffle system for permitting fresh air to flow into the building. The baffle system includes movable baffles disposed for closing openings formed in the building, which openings are located in the ceiling or in the sidewalls adjacent the upper edges thereof. A drive mechanism is connected to the baffles for moving same between open and closed positions. The drive mechanism is actuated by a pressure sensing device which senses the pressure differential between the pressure on the inside of the building and the atmospheric pressure which exists outside the building. The sensing device maintains the pressure in the building at a level which is slightly below atmospheric pressure.