Microiontophoresis is a method which allows to administer neuroactive compounds like transmitters or antagonists in very small amounts to small circumscribed volumes of neuronal tissue. To do so, the charged compounds are injected into the target area by DC currents of appropriate polarity which cause the charged molecules to migrate within the electrical field (Fig.). In combination with electrophysiological methods, microiontophoresis can be used to study effects of pharmacological manipulations on a single neuron level without systemic drug administration.