The detailed schematic diagrams and construction techniques are presented for a single microelectrode voltage clamp. The devices is used to study the membrane processes of small cells not able to be penetrated with the traditional two microelectrode system. The technique utilizes the same microelectrode alternately for current injection and membrane potential sampling on a time-sharing basis controlled by electronic switching circuitry. Current is injected in pulses and the membrane potential is sampled after an individual current pulse discharges from the microelectrode capacitance to the true membrane potential. The device can either measure the voltage response to an injected current waveform (current injection mode) or the membrane currents generated during a controlled change in membrane potential (voltage clamp mode). In voltage clamp mode, the membrane potential reaches steady-state within 2 msec (maximum time) in response to a 40 mV step command. The single electrode voltage clamp is potentially very important to the investigation of slow current processes within electrically excitable cells too small to be previously studied with traditional voltage clamp technology.