An objective evaluation of various combinations of donor andreceptor solutions intended for permeation of actives through sili-cone membranes is presented. The concentration of PG in variousbinary mixtures of donor solution appeared to influence the rate ofpermeation of two actives, namely lidocaine and salicylic acid. Thekey finding was the influence of receptor medium on the rate oftransport of actives through silicone membrane. This informationis useful in the rational design of an in vitro diffusion test system fortransdermal formulations and highlights the importance of recep-tor phase choice since receptor phase-membrane interactions areinevitable.