A simple, battery-powered current source is described that is suitable for the iontophoresis of axonal tracing substances, such as Phaseolus vulgaris leucoagglutinin. Unlike the previous designs that form the basis of most commonly used iontophoretic devices, this circuit does not use operational amplifiers to provide controlled current. Instead, a pair of bipolar transistors that can be selected to provide a compliance of many hundreds of volts form the regulating circuitry.