The aim of the present study is to validate different measures for mental workload. Ten aspirant fighter jet pilots flew several scenarios in a flight simulator. The scenarios were divided into segments with different levels of task load. During the flight, heart rate, respiration and blood pressure were recorded. After the tasks, subjects rated the work-load for segments of the scenario with the NASA-TLX and the BSMI (a Dutch effort scale). Furthermore, cortisol was measured on the training and experimental day (before and after each scenario). Heart rate, heart rate variability and several respiratory parameters reflected the changes in task load. Blood pressure was not systematically affected by task load. The BSMI was more sen-sitive than the TLX; the task load effect was stronger and the correlations with the physiological parameters were higher. Cortisol levels were slightly increased on the experimental day but the post task levels did not differ from the pre-task levels. There was a correlation between mean cortisol levels and performance: subjects with high cortisol levels performed poorly.