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The feasibility of using presently available liquid lubricants in advanced, high speed aircraft engines has been investigated with a recirculating inerted lubrication system. Three fluids performed satisfactorily for short durations of 3-10 hr in a full-scale simulated aircraft bearing and seal assembly at an outer race bearing temperature of 700 F and with a bulk fluid temperature of 500 F. The principal problem has been with excessive leakage of the oil side bellows face seals which has accounted for a majority of the test terminations. With only one lubricant (a MIL-L-7808E oil) could bearing failure be blamed on a lack of lubricating ability. An inerted oil mist once-through system was also tested, but with less promising results with only one lubricant running successfully at 600 F bearing temperature. The mist system appears to be limited by its inability to maintain stable bearing temperatures and requires further development.