A conceptual framework is presented in which mental load and stress ar
e regarded as two distinct biobehavioural states in the work environme
nt that differ in energy mobilization, mood and coping strategy. The f
ramework combines two types of theory that are based on human performa
nce research using laboratory tasks on the one hand and applied resear
ch in the work environment on the other. A high workload is regarded a
s an important but not a critical factor in the development of stress
symptoms. Even under unfavourable conditions it is possible to work in
tensively and to be highly activated without feelings of strain or psy
chosomatic complaints. In contrast, working conditions that provide fe
w possibilities for control and little social support or are associate
d with reduced well-being and increased health risks. The two states d
iffer in activation, coping style and mood. A proper distinction is im
portant not only for theory building but also for the reconstruction o
f the work environment. It may lead to recommendations that aim to enh
ance the work efficiency of employees while reducing the probability t
hat stress responses will occur.