The effect of biologically active compounds on ion channel transport within a self-assembled lipid monolayer system was investigated by examining interactions between some bioactive compounds and the gramicidin-modified lipid monolayer as a membrane model in solutions of different cation concentration and composition. Techniques used were ac voltammetry to measure the capacitance of the layers and cyclic voltammetry (CV) and chronoamperometry to study Tl electrochemistry. The CV results tend to support the proposed model that nonlinear diffusion effects are not significant in the electrochemistry of Tl within the time scale of the experiment. As a result the electrode process is treated as a CE mechanism where the rate limiting step is the translocation of the ion in the channel. Complications arise because there is competition between the electroactive ion, Tl+, and the univalent electrolyte ion to enter the channel under the influence of the applied potential field. Values of the permeability rate constants have been derived from the electrochemical data. Results indicate that many hydrophobic additives in the lipid layer in particular polyaromatic and polyconjugated compounds selectively give rise to an alteration in the permeability of gramicidin-modified DOPC monolayers to Tl+. Up to 5-fold increase in gramicidin-mediated permeability is noted in monolayers with added retinol, which is similar to that seen in the presence of a similar concentration of negatively charged lipids (PS) in the layer. The chlorine-substituted aromatic pesticide DDT causes an apparent depression in the permeability. It is proposed that the variations of permeability to Tl+ due to additives in the layer are predominantly caused by the effect of the compounds within the monolayer on the energy barrier to the translocation of the ion within the channel. This occurs because the compounds alter the image forces acting on the ion passing through the channel as well as directly interacting with the ion itself.