A device has been designed and constructed, largely of commercially available operational amplifiers, to measure the voltage‐current relations in biological membranes due to the ion flux through them. The device operates in three modes: to clamp the voltage across the membrane to some predetermined value and record the current, to clamp the current through the membrane and record the trans‐membrane voltage, and to recorddV/dI, the resistance of the membrane measured in a time short compared to the ion redistribution time.