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Potential differences have been measured during transdermal iontophoresis in order to establish the effect of voltage, as opposed to current, on cutaneous blood flow. It is known that, even in the absence of drugs, the iontophoresis current can sometimes produce increased blood flow. The role of voltage in this process is studied through single-ended measurements (between electrode and body) of the potential difference during iontophoresis with 100-μA, 20-s current pulses through deionized water, saturated 20.4% NaCl solution, 1 % acetylcholine, and 1 % sodium nitroprusside. It is found that the voltage needed to deliver the current varied by orders of magnitudes less than the differences in the conductance of these different electrolytes, and it is concluded that, at least for the present current protocol, the voltage as such is not an important factor in increasing the blood flow.