A system for precise recording of eye position and movements in laboratory animals, by means of Hall-effect devices, is described. The system, useful in neurophysiological and neurobehavioral studies, allows the analysis of saccadic eye movements, optokinetic-and vestibular-induced nystagmus, slow tracking movements, eye vergences, and so forth. This small, light-weight, and inexpensive system uses a set of Hall-effect devices and associated electronics to sense variations in the position of high-power magnets fixed in the eye sclera or in scleral contact lenses. The output of the Hall-effect devices is amplified by operational amplifiers, collected through an AID converter, and analyzed in a PC computer by specific software.The systems for tracking eye movements and positions have a wide application in different research and clinical areas, such as neurophysiology, psychobiology, or clinical and experimental neurology. A number of different methods for measuring eye movements have been developed to date, such as electro-oculography (Watanabe & Takeda, 1996; Young & Sheena, 1975a), the search coil technique (Robinson, 1963;Schlag, Merker, & Schlag-Rey, 1983), infrared light corneal or scleral reflecting systems (Bach, Bouis, & Fischer, 1983;Eizenman, Frecker, & Hallett, 1984;Reulen et al., 1988;Young & Sheena, 1975b), videooculography (Discenna, Das, Zivotofsky, Seidman, & Leigh, 1995Ott, Gehle, & Eckmiller, 1990), and laserophthalmoscopy Ott, Lades, Holthoff, & Eckmiller, 1990). Each of these techniques presents advantages, limitations, and inconveniences. Thus, among the most widespread methods, electro-oculography presents the advantage ofeasy application and lower cost, but also limitations of bandwidth, low stability, and contamination by nonocular events. The search coil technique presents long-term stability and high resolution but some inconveniences, such as the difficult ocular coil implantation, the presence of wire leads, the interference of alternating magnetic fields on the simultaneous recording of other neurophysiological variables, and high cost.The present paper describes an inexpensive system using Hall-effect sensors for precisely measuring the horizontal and vertical components of eye movements such as saccades, optokinetic-and vestibular-induced nystagmus, slow tracking movements, eye vergence, and so forth. The Hall effect consists in the generation of a voltage across an electrical conductor carrying current when it is placed in a magnetic field. Thus, the Hall-effect sensors produce an output voltage that changes in proportion to the intensity ofthe surrounding magnetic field. In the present application the magnetic field is provided by a small magnet attached to the eye sclera. The system described here has some advantages over existing tracking methods, because it provides a degree ofprecision, resolution, and dynamic range similar to the most precise and widely used methods, but is small, light weight, and technically simple. In addition, this system allows the recording of eye movem...