A system for monitoring impedance of excitable cells in vitro, which includes a device for monitoring cell-substrate impedance at 20 millisecond resolution, which includes a nonconductive substrate with one or more electrode arrays fabricated in one or more wells, wherein cell attachment on the substrate can result in a detectable change in impedance between electrodes within each electrode array; an impedance analyzer capable of impedance measurement at 20 millisecond time resolution; electronic circuitry that can engage said device and selectively connect said two or more electrode arrays of said device to said impedance analyzer; and a software program that controls said electronic circuitry and records and analyzes data obtained from said impedance analyzer.