When rendering immersive virtual environments, it is necessary to limit the geometric complexity of the scene in order to retain interactive frame rates. Both reduced frame rendering rates and reduced geometric complexity have been shown to have a negative impact on task performance. We present a perceptually adaptive rendering system which optimizes rendering performance in immersive virtual environments. Our system currently uses three discrete levels of detail for each model, which are created prior to run time. We use head position and orientation in the 3D environment to determine the appropriate geometric level of detail (LOD) of the objects in the scene. Based upon the thresholds used to trigger a change between two LODs, interactivity can be significantly improved without overtly distracting the user.