GPS collars have the potential to automatically collect large numbers of relatively accurate animal relocations. Collar costs, levels of accuracy, and satellite signal reception have been reported by other studies, but there has been little discussion of long-term performance under field conditions. Between March 1996 and April 1999, we placed 11 GPS collars on 23 individual woodland caribou Rangifer tarandus caribou for a total of 26 collar deployments. Reliability was highly variable; some collar deployments operated normally for their expected period of time, other deployments functioned for less than half of their expected lives. Collars attempted 41,822 locations and collected 15,247 3-D and 10,411 2-D locations, for an average acquisition rate of 59%. We recommend that researchers carefully consider project objectives, budget constraints, and available options such as differential correction and remote collar communication, before purchasing GPS collars.