Abstract: Tiers of cages for animals to be raised or tended are disposed in opposing orientation at opposite sides of an elongated room. The tiers are spaced above the floor of the room and inward from the opposite sidewalls. The ceiling of the room has a multiplicity of separate vent apertures spaced apart lengthwise of the room with sliding doors for adjusting the effective open area of the apertures. Exhaust fans are recessed in the ceiling of the room and are effective to expel air from the room to the exterior of the building. Expelling air from the room reduces the pressure and outside fresh air is drawn into the room through the vent apertures. Opening and closing of the vent apertures is coordinated with operation of the exhaust fans so at to maintain a desired negative pressure in the room, which results in fresh air being injected into the room as downward extending jets.