A pupillometric system was developed to be used at the Neuroscience and Behavior Research Group from Universidad Industrial de Santander. First of all, an acquisition system (camera) was selected and implemented, taking into account that pupil diameter varies from 1.5 mm up to 8 or 9 mm, reacting to stimuli in 0.2 seconds. Then, the selected camera was characterized, a measurement and calibration protocol was implemented, and an algorithm for measuring pupil diameter was established. The system gives pupil diameter as a function of time, with an accuracy of ± 0.2 mm.