Background. Pupil size is mainly controlled by the action of parasympathetic and sympathetic nerves. Measurement of pupil size and reaction to light are important in both experimental and clinical settings. Methods and Results. We have established infrared pupilometry using a commercially available digital camera and a calculation with standard computer software. Volunteers were habituated in a dark room for five minutes before pupil evaluation. Pupil measurement was repeated three times and the diameter was calculated according to the following formula: D(mm)=30/S(pix)*D(pix), where D(pix) and D(mm) represent the pupil size in pixels and millimetres, respectively, and S(pix) was the length in pixels of a 30 mm standard. Our method was validated by the measurement of the diameter of calibration circles printed on a white sheet of paper. Deviation from the actual size was less than 3 %. We also compared the results of pupil size obtained by our method, with the results of the pupilometer Pupillscan II measurement. The relative difference between the two methods was always less than 5 %. The applicability of the method was illustrated by measurement of the pharmacodynamic effect of the single dose of tramadol (100 mg p.o.) in seven volunteers, compared with control placebo-treated volunteers. Conclusions. The proposed method is precise and sensitive enough to be used for pupil size determination.

        Key words: pupil, miosis, pharmacokinetics, pharmacodynamics, tramadol.