The spontaneous fluctuations of voltage across a glass microelectrode have been studied as a function of the average difference in voltage (V) across the electrode and the concentration of internal (n 2 ) and external (n 1 ) salt solutions. A stationary fluctuation was observed, with a mean square value many times greater than that calculated from the Nyquist formula for noise due to the thermal agitation of charge, for all states of the electrode except n 2 =n 1 , V=0. Here the fluctuations equal the Johnson level. The spectral density of the fluctuation and its dependence on (n 1 , n 2 , V) are given in the range 3-0.003 MKCl and V = ±200 mV. The dependence on the species of cation (potassium and sodium) was also determined. The work has practical implications for users of microelectrodes who wish to minimize noise. It also serves as a model for studying noise generated when ions move through a micropore.