The development and effective use of robust high-electrode-count microelectrode arrays for neuronal recording and stimulation depends on effective monitoring of electrode impedances and how these change over time. In multielectrode arrays, conventional electrode impedance measurements may be confounded by possible shunting of signals among electrodes. Additionally, most present methods to monitor impedances in high-electrode-count arrays are labor intensive, requiring manual testing of one individual electrode at a time. We have developed a system capable of automatically measuring the impedances of each microelectrode on a 100-microelectrode array with a 1-kHz, 10-mV sine wave. Through switching logic, two impedance values are measured for each electrode in an array: (1) the unshunted impedance (presumably representing the actual tip impedance); and (2) the shunted impedance. These two measurements are used to calculate the net impedance of leakage/shunting pathways from the test electrode through all the other electrodes in the array. The system measures impedances in the range of 300 Omega-10 MOmega. The system was validated with simple resistor ladder networks, and measurements of the modeled electrode tip impedances were within 2% of independently measured values. Additionally, the system reliably indicated the relative values of the net shunting impedances, although high values were systematically underestimated. The automated device was capable of measuring electrode tip and net shunting impedance values for a 100-microelectrode array in 5 min. These rapid and repeatable measurements allow for the quantitative assessment of high-electrode-count arrays over time.