Closed-loop systems, where neural signals are used to control electrical stimulation, show promise as powerful experimental platforms and nuanced clinical therapies. To increase the availability, affordability, and usability of these devices, we have created a flexible open source system capable of simultaneous stimulation and recording from multiple electrodes. The system is versatile, functioning with both freely moving animals and in vitro preparations. Current- and voltage-controlled stimulation waveforms with 1 micros resolution can be delivered to any electrode of an array. Stimulation sequences can be preprogrammed or triggered by ongoing neural activity, such as action potentials (APs) or local field potentials (LFPs). Recovery from artifact is rapid, allowing the detection of APs within 1 ms of stimulus offset. Since the stimulation subsystem provides simultaneous current/voltage monitoring, electrode impedance spectra can be calculated in real time. A sample closed-loop experiment is presented wherein interictal spikes from epileptic animals are used to trigger microstimulation.