The transport of nanoparticle associated drugs, [Se-75]norcholestenol, captopril, methylene blue, hydrocortisone, doxorubicin, and dalargin was determined by permeability measurements in two chamber side by side diffusion cells using cellulose acetate, silicone rubber, pig small intestine, or hairless mice skin as membranes. Solutions of free drugs served as controls. The permeabilities depended on the physico chemical properties of the drugs which governed both, drug interaction with the nanoparticles as well as with the membranes. Consequently, the influence of dilution of the nanoparticle or free drug preparations on permeabilities was complex. With the exception of [Se-75]norcholestenol the permeabilities were higher with free drugs than after binding to nanoparticles. The permeabilities of the membranes decreased in the order cellulose acetate, pig small intestine, silicone rubber, and hairless mouse skin. (C) 1999 Elsevier Science B.V. All rights reserved.