We have studied the effect of polyethylene glycols (PEGs) on the iontophoretic flux of acetaminophen (AAP) using conventional in vitro iontophoresis methodology. A series of PEGs with average molecular weight (MW) ranging from about 100 to 1,500 was studied. The results were analyzed to explain how PEGs affect the electroosmosis and flux through skin. As a marker molecule for the direction and magnitude of electroosmotic volume flow (EVF), AAP was used. PEG decreased both anodal and cathodal AAP flux markedly. The magnitude of this decrease in flux increased as the MW and the concentration of PEG increased. From the Helmholtz-Smoluchowski equation, it was expected that the increase in viscosity and the decrease in dielectric constant are thought to be the main reason for the decrease in EVF and the flux. The large increase in solubility of AAP in PEG solution may also play an important role, because this increase lowers the partition of AAP into the stratum corneum. When 30 % diethylene glycol solution was used, the magnitude of EVF was estimated to be about 1.5 μl/cm(2) h, and it decreased as the MW of the PEG increased. These results and discussions clearly suggest that the incorporation of organic solubilizers and penetration enhancers into the iontophoretic formulation should be carefully decided after a thorough understanding of their effect on flux. Overall, these results provide further mechanistic insights into the role of electroosmosis in flux through skin, and how they can be modulated by PEG and their MW.