With the view to determining whether or not gas entrained in the sodium coolant could cause overheating of a fast reactor core, the following items were studied: Equations were derived to serve in estimating the thermal-hydraulic effect of the gas entrainment, and calculations performed therewith to obtain information on conditions corresponding to the Core A under operation in the Fermi Reactor. The results of the present examination reveal that in the Fermi Reactor an amount of gas almost inconceivable as a practical possibility must be entrained before the coolant or the fuel surface would be heated to the boiling point of sodium.