Ions formed by collision in electronic amplifier tubes move toward the cathode and control‐grid and produce momentary increases in the space‐charge limited current (current pulses). These current pulses are random in time and produce fluctuation noise analogous to the shot‐effect. An approximate theory is presented, applicable to cathodes of large diameter (ca.0.1 cm). Both the noise and the increase in the average value of the electron current due to the presence of ionization are calculated in terms of the ``size of the elementary event'' or the integral of the current pulse. It is not feasible to calculate the latter directly but it may be evaluated indirectly by measurements of the increase of average current. The noise calculated from the values so obtained and from the ionization probability data of Smith, Bleakney and Tate is in close agreement with that observed experimentally. The production of noise in tubes with oxide cathodes containing mercury vapor, argon and the gases naturally evolved from the electrodes and tube walls is investigated experimentally as affected by pressure and electron current density. In the case of a tetrode containing mercury vapor or evolved gases the variation with pressure is linear up to 10−3mm; in the case of argon the noise varies as the 1.1 power of the pressure. In the mercury vapor tetrode a variation as the 3/2 power of the plate current was found, as compared to a 5/3 law, expected theoretically. These measurements were made at 620 kilocycles. The noise per frequency interval was measured as a function of frequency over the range 500–1500 k.c. At pressures of the order of 10−4mm in mercury vapor a decrease at the higher frequencies was observed; with argon the noise was uniform. With mercury vapor at higher pressures (ca.4×10−3) the noise spectrum became peaked, the frequencies of the peaks depending upon the electron current flowing (electrode potentials). The hypothesis that these peaks are due to oscillations of positive ions in the potential trough surrounding the cathode is in accord with the principal experimental facts. With further increase in pressure continuous oscillations are produced, sustained apparently by a regenerative action of the space charge upon the oscillating ions. The frequencies of these oscillations are affected only by the electrode potentials and not at all by the external electrical circuit.