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 field study of the implementation of a command-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 findings are discussed in terms of Turner and Karasek's integrated model of the relationships between computer 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.