There have been many attempts to define the key relationships between passive drug diffusion across the skin and the molecular and physicochemical properties of the permeant. At the present time, the importance of lipophilicity (or hydrogen bond donor and acceptor properties) and of molecular volume are well established, and useful predictive relationships for passive percutaneous permeability exist. With respect to iontophoresis, on the other hand, the situation is far less clear and the mechanisms involved have not been completely defined. The roles of electromigration and electroosmosis (current-induced convective solvent flow) are now beginning to be understood and experimentally separated. In turn, this allows the manner in which certain physicochemical parameters influence the efficiency of drug electrotransport to be deduced. An initial examination of a database drawn from the literature and from our own work (for which the experimental conditions employed were reasonably constant) suggests a rather sharp dependence of cationic drug delivery via electromigration upon molecular size. We suggest that the analysis reveals useful paths for further investigation.