In mountainous terrain, performance of VHF telemetry is often impaired by long distances between observer and animal, and by reflections of radio signals. GPS telemetry should be less affected by such constraints, but its performance has not been sufficiently tested in areas of rugged topography. We studied how current GPS telemetry deployed on red deer Cervus elaphus performs in the strong topographic relief of a rugged Alpine landscape. We examined the influence of topography, vegetation and red deer activity on GPS performance, assessed the accuracy of 2D positions of red deer and compared differences in GPS performance between collars in trials and collars fitted to deer. GPS performance was characterised by consistently high position acquisition rates (PAR), but a relatively low proportion of 3D positions compared to smoother terrain. The proportion of 3D positions was lower in forest than in open land but within forest different vegetation structures had only a weak influence. For collars fitted to red deer, we found that > 70% of the positions were either 3D positions or accurate 2D positions. The collars produced higher PAR and proportions of 3D positions in the trials than when they were fitted to red deer, probably reflecting an effect of red deer behaviour. We also observed better performance of collars on red deer during night than during daytime, because animals were more active at night where they more often occurred in open land. An effect of season on the proportion of 3D positions was found in collars fitted to red deer, but not in the trials, and may be interpreted as a secondary effect produced by seasonal differences in habitat selection by red deer. The fact that PAR was not affected more strongly by the rugged mountainous relief probably reflects the recent progress achieved in GPS technology, whereas the comparatively low proportion of 3D positions illustrates that the probability of four satellites being available at anyone time is still constrained by topography.