Fast-scan cyclic voltammetry is a unique technique for sampling dopamine concentration in the brain of rodents in vivo in real time. The combination of in vivo voltammetry with single-unit electrophysiological recording from the same microelectrode has proved to be useful in studying the relationship between animal behavior, dopamine release and unit activity. The instrumentation for these experiments described here has two unique features. First, a 2-electrode arrangement implemented for voltammetric measurements with the grounded reference electrode allows compatibility with electrophysiological measurements, iontophoresis, and multielectrode measurements. Second, we use miniaturized electronic components in the design of a small headstage that can be fixed on the rat's head and used in freely moving animals.