Reverse iontophoresis is used by the GlucoWatch® Biographer to noninvasively extract glucose across the skin, allowing a diabetic's glycemia to be evaluated every 10 min over several hours. However, before each use, the device must be calibrated with a blood sample assayed in the conventional way. The objective of this study was to identify an approach by which to avoid this invasive step. The dermal (anodal) side of porcine skin in vitro was bathed in buffered (pH 7.4) solutions containing glucose, at concentrations from 3 to 10 mM, and physiological levels of sodium chloride. Constant current was applied and the cathodal solution contacting the outer skin surface was analyzed periodically for the quantities of Na⁺ and glucose extracted by “reverse” electromigration and electroosmosis, respectively. Although the extracted Na⁺ flux was invariant, as expected, given the essentially fixed NaCl concentration present in the physiological system, the glucose samples reflected proportionately the subdermal concentration. Equally, the extracted flux ratio (glucose/sodium) varied linearly with the subdermal glucose/sodium concentration ratio; knowing the gradient of this correlation, therefore, means that a measurement of the extraction flux ratio can be used to determine the subdermal glucose concentration (the physiological [Na⁺] being known and fixed). Thus, a refinement of the reverse iontophoresis technology using the simultaneous determination of the extracted fluxes of the analyte of interest (glucose) and of an “internal standard”, whose level in the biological system is invariant (Na⁺), may permit a noninvasive sampling methodology free of the need for calibration with a blood sample.