Abstract: A high density animal housing system with an air source conduit and an air exhaust conduit is normally coupled to one or more ventilating animal cage racks through standard inlet/outlet fittings. A euthanasia fixture is coupled into the standard inlet/outlet fittings and selectively and sequentially operates valves and/or blowers to switch from supply of respiration air to a gas supply, to open and close the flow to the exhaust and to resume ventilation afterwards, for venting. The sequence is timed and controlled by a programmable controller that activates the associated blowers and valves automatically to follow a user selected sequence. The system anesthetizes and then euthanizes the animals via the same flow conduits that otherwise supply respiration air, requiring no rack modifications and little if any human attention other than to couple the rack to the ventilation system at the euthanasia fixture. Operator inputs allow selection among sequences.