Direct recordings of electron currents mediated by cytochromes b561 are not available yet, despite the importance of these proteins in a variety of physiological functions including neurotransmitter synthesis and dietary iron uptake. Here, we used the two-electrode voltage-clamp technique applied to Xenopus oocytes to demonstrate, for the first time, the generation of electron currents by a Drosophila member of the CYB561 superfamily named SDR2 (stromal cell-derived receptor 2). This experimental method, along with the theoretical development of a three-state kinetic model, leads to the hypothesis that electron donor/acceptor concentrations and trans-membrane voltage mutualistically control SDR2-mediated electron transport activity in a complex but predictable manner.