Noise spectra of the drain current of ion selective field effect transistors in equilibrium were measured. The spectra differ from one type of ISFET to the other. The ionic activity in solution does not modify perceptibly the noise level for any of the various types of ISFET's. Computer assisted modeling is used to fit experimental data and to assign an equivalent electrical circuit to the electrochemical system. The overall response time of the system is shown to depend on the input capacitance of the electrometer.