Abstract: Methods and apparatus are provided which allow for closed-circuit ventilation for the treatment or diagnosis of disorders. The closed-circuit ventilation apparatus of the present invention provide a closed-circuit respirator that isolates the gas flow path from the ventilator apparatus. This allows the prolonged administration of expensive materials such as fluorochemicals without excessive loss due to evaporation. As such the provided methods an apparatus are particularly applicable to liquid ventilation including partial liquid ventilation and total liquid ventilation.