We present quantitative measurements of the adsorption of drug ions at Langmuir monolayers, which are considered as a model for biological membranes. The technique we have employed is based on scanning electrochemical microscopy using a capillary probe filled with organic electrolyte solution and operated in an inverted submarine arrangement in a Langmuir trough. In this experimental arrangement, we have made potential step chronoamperometric measurements, with the probe adjacent to the Langmuir monolayer deposited at the air/water interface, to extract information on the adsorption process. We have measured the adsorption of a cationic beta-blocker drug, propranolol, at a Langmuir monolayer of stearic acid. By developing a theoretical simulation that assumes that the equilibrium at the interface is controlled by the competition between the acid/base equilibrium of the stearic acid molecules at the surface and the electrostatically induced adsorption of the positively charged propranolol ion, we have been able to extract from our data a drug adsorption constant of (5 +/- 1) x 10(-5) mol dm(-3) at the monolayer surface pressure of 10 mN m(-1).