Introduction. Impulse conduction in demyelinated central axons may fail as the body temperature rises because faster Na+ channel kinetics give rise to briefer action potentials, and a temperature dependent membrane potential moves the resting potential away from threshold (eg Coates et al 2015). Both mechanisms might contribute to conduction failure where the safety-factor for impulse conduction is lowered. Aims and methods. We have investigated the effects of applying infra-red light, generated by a laser-diode (1550 nm), to undamaged rat optic nerve, via a 9 mm diameter fibre-optic light guide. Optic nerves were isolated from Wistar or Sprague Dawley rats following a Home Office approved schedule 1 procedure, and maintained in a nerve-bath. Axon excitability was repeatedly measured in response to a constant-current stimulus delivered via a suction electrode, using the technique of computer controlled threshold-tracking, and maintaining a 50 % maximal response. Changes in axon membrane potential were recorded across a grease gap. Results. We report that the application of IR laser light reversibly increased threshold (400 mA diode driving current, n = 3, p = 0.038, one sample t-test, Fig. 1a). At constant light energy (350 mA driving current), the change in threshold was significantly raised by 99.81 ± 32.93 % following the application of 50 mM ZD7288, a blocker of Ih, (n = 5, p = 0.029, paired-t-test). These results are consistent with the laser light causing axonal hyperpolarization, that we have also recorded directly (Fig. 1b). Conclusion. At least a part of the action of IR laser light on optic nerve is associated with locally raised temperature and a membrane potential hyperpolarization, explained by the previously hypothesized role for electroneutral Na+ entry into optic nerve axons (Coates et al 2015; Kanagaratnam et al 2017). Fig.1 IR light affects optic nerve. a) Laser light application (400 mA driving current) raised current threshold by 7.01 ± 1.42% (mean ± SEM).  b) Temperature fluctuations in flowing buffer solution (lower panel) mimic laser light application (350 mA driving current) on axon membrane potential (upper panel).