After chemical stimulation of the human olfactory epithelium it is possible to record a negative response (electro-olfactogram, EOG) which is interpreted as the summated receptor potentials of the olfactory nerve. The aim of the present investigation was to test the EOG's changes in relation to pairs of stimuli. Stimulation was performed with vanillin (0.8 ppm) regarded to exclusively excite fibers of the olfactory nerve. Ten healthy volunteers participated in the experiments. Pairs of stimuli were applied at different interstimulus intervals ISIs (2-8 s). EOG could be recorded in 6 out of 10 subjects. After olfactory stimulation the responses' peak amplitude was found to range from 0.15 to 1.8 mV. When pairs of olfactory stimuli were applied responses obtained at an ISI of 8 s were clearly separated whereas at an ISI of 2 s responses were superimposed on each other. As with an ISI of 8 s, the amplitude produced by the second stimulus was nearly as great as the first responses' amplitude (decrease by approximately 20%). In contrast, intensity estimates obtained in an additional experiment (n = 10) decreased by 40-60%. Based on the present data peripheral encoding in the olfactory system appears to be less subject to desensitization compared to the decrease of intensity estimates.