Relevant future applications of ISFETs cannot be discussed without considering the original research, development and application aims and the corresponding results as achieved up to now. The author’s original intention in developing the ISFET was to measure the ionic sodium influx and potassium efflux at nerve and muscle membranes. The silicon dioxide of a MOSFET, from which the gate metal was etched away, appeared to be sensitive for sodium and potassium ions, and thus well suited for the intended application. Sensitivity to pH was also observed, but was not seen as directly of interest for the electrophysiological experiments. The non-selectivity of silicon dioxide has recently been confirmed by independent measurements. With respect to the measurement of ion fluxes, it should be noted that the related current densities are in the order of some rnA/cm’, resulting in ionic transports in the order of picomoles/cm2. However, by catching the ionic flux in a very small cavity, which is achieved by pressing the ISFET with its surrounding ring of encapsulant firmly against the nerve membrane, the ionic changes may rise to 0.1 millimol/litre, easily detectable by an ISFET. The recorded monophasic pulses are therefore most probably indeed the result of very local ionic concentration changes, although this has never been proved in any later paper. In the context of the present paper, it is important to notice that in the electrophysiological ISFET application as mentioned above, the fast response of an ISFET is explicitly exploited, combined with its small dimensions.