The narcotic analgesic, hydromorphone, was delivered by constant-current iontophoresis from aqueous solution through excised pig and human skin and from a hydrogel formulation into domestic pigs. The delivery rate per unit current was found to be similar for both pig and human skin, with a value of 1.1 mg h −1 mA −1, even though the passive fluxes differed by a factor of six. The in vitro delivery rate through pig skin at a current density of 125 μA/cm 2 was found to be independent of the concentration of hydromorphone in the donor solution over the range from 0.01 M to 0.8 M. No correlation was observed between the initial passive hydromorphone delivery rate through pig skin and the steady-state rate during iontophoresis. In addition, in vivo delivery of hydromorphone into domestic pigs was studied at currents of up to 1.2 mA for 12 hours. Delivery rates were determined from plasma hydromorphone concentrations and from residual drug analysis of spent patches. The delivery rate per unit current determined from the plasma concentration and residual assay data were 1.9 and 1.2 mgh −1 mA −1, respectively.