The effects of various changes in both tube and circuit conditions have been investigated with regard to their influence on the limitation which fluctuation noise sets on the sensitivity of a receiver. It is concluded that for minimum noise the following conditions should obtain: The gas pressure in the tube should be less than 10-4mm of mercury; the antenna-to-grid transfer circuit should be as efficient as possible; the plate-circuit load impedance should be high enough to give a gain of at least five for the first radio-frequency tube apart from the antenna coupling circuit; and the cathode emission should be high enough so that the tube is always operating under dense space-charge conditions.