The stressful nature of computer-based work is often highlighted in the research literature. In this study, we argue that a well designed computer system should realize the twin aims of enhancing performance and lowering stress. This paper reports on a psychophysiological ® eld study of the implementation of a comm and-and-control system in an ambulance service. The evaluation revealed both improvements in operator performance and a reduction in stress levels. In particular, it was found that computer support reduced both systolic blood pressure and subjective anxiety during conditions of peak workload. These ® ndings are discussed in terms of Turner and Karasek' s integrated model of the relationships between com puter system design, task performance and well-being. The success of the computer system was attributed to the support that it gave operators; by enhancing their degree of control it enabled them to cope better in a highly demanding work environment. The study shows that psychophysiological techniques have a valuable role to play in system design/evaluation; and more generally, that systems development methodologies should take greater account of applied psychological research, especially in areas such as stress.