Electric current through a cathode in the form of a metal pipe with electrolyte flowing inside is studied experimentally. The electric current is measured at different flow rates for two liquids-highly conducting water, and oil with low conductivity. The results show the electric current saturation at high voltages in water and no saturation in oil. The physical origin of the electric current saturation in electrolytes is still debated. In macroscopic channels the saturation was once attributed to the diffusion-limited mechanism proposed by Levich. However, the saturation observed in the present experiments cannot be described by Levich's result and has a different nature. No other explanation of the electric current saturation in macroscopic channels is available, as to our knowledge. Our theoretical interpretation relates the electric current to a first-order irreversible faradaic reaction at the cathode surface and explains the dependence of the current density on the ion concentration and the electrolyte flow rate. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.015210jes] All rights reserved.