Although electrically assisted transdermal drug delivery has recently achieved a great deal of research attention, the precise anatomical pathway followed by these drugs through the stratum corneum has not been clearly defined. Pigs are an accepted model for studying iontophoretic drug delivery in humans. The purpose of this investigation was to visualize the pathway of ion transport by iontophoresing mercuric chloride. Weanling Yorkshire swine were dosed with 7.4% mercuric chloride in the positive electrode at a current density of 200 µAmp/cm2 applied for 1 hr. Biopsies were immediately taken, exposed to 25% ammonium sulfide vapor to precipitate and localize the mercury, fixed, and processed for light and transmission electron microscopy. The presence of mercury, which appeared as a black precipitate, was confirmed using energy-dispersive X-ray microanalysis. Although some compound penetrated the skin through appendageal pathways, the electron micrographs clearly revealed that mercuric chloride traversed the intact stratum corneum via an intercellular route. Precipitate was also localized in the outer membrane of the mitochondria in the viable epidermal cells, dermal fibroblasts, and capillaries, demonstrating transdermal delivery and systemic exposure to the mercury. These findings have implications for iontophoretic drug delivery, since they allow visualization of the functional “pores” predicted by mathematical models.