Two experiments are reported; experiment I on performance decrement during prolonged night driving and experiment II on driver stress during driving in different environments.In experiment I, Subjects in one condition carried out a continuous driving task during the night which was preceded and followed by a short drive, while in another condition subjects only carried out the pre- and post tests and slept in-between. The results showed considerable decrement in driving performance (a.o. increase in lane drifting) and performance on two subsidiary tasks, while heart rate, although decreasing during the drive, could not be directly related to fatigue, but was also influenced by adaptational effects.In experiment II, Subjects drove three different short test routes during daytime: in town, on a four-lane motorway, and on a two-lane rural road. Measurements were taken of driving quality (a.o. lane positioning and steering wheel angle), heart rate and reaction time on an auditory subsidiary task. Besides, being sensitive enough to differentiate between different levels of environmental complexity, heart rate variability measures seemed very promising in facilitating the interpretation of driver behavior measures. Also, there existed some remarkable differences in lateral and longitudinal control between motorway and rural road.