This paper presents a study of the fine grain physiological responses of participants to an immersive virtual simulation of an urban environment. The analysis of differences in participant responses at various stages of the experiment (baseline recordings, training, first half and second half of the urban simulation) are examined in detail. It was found that participants typically show a stress response during the training phase and a stress response towards the end of the simulation of the urban experience. There is also some evidence that variations in the level of visual realism based the texture strategy used was associated with changes in mental stress. Categories and Subject Descriptors I.3 [Computer Graphics]: Three-Dimensional Graphics and Realism - Virtual reality; J.4 [Computer Applications]: Social And Behavioral Sciences - Psychology General Terms Human Factors Keywords Immersive Virtual Environments, Physiology, Human Response, Evaluation