[en] The aim of this study was to explore the role of expectancies and beliefs about the potential effects of electromagnetic fields (EMFs) (what the subject thought the effect was going to be) and the effects of 50 Hz magnetic fields (400 microT(rms)) acute exposure on cognitive performance, the reporting of physical symptoms and some psychological and physiological parameters. Seventy-four healthy male volunteers aged between 40 and 60 years of age were randomly assigned to one of five groups, which differed in (1) the type of information they were given concerning the expected magnetic field effect on performance in cognitive tests (positive = enhancement of the performance; negative = impairment of the performance; neutral) and (2) the type of exposure (real or sham). Three groups were sham exposed with positive (group+), negative (group-) and neutral information (group+/-); one group was really exposed with neutral information (group expo) and one group was not exposed, though they wore the helmet, and did not receive any field-related information (control group). All the volunteers, except the control group, were led to believe that they would be exposed to a magnetic field of 400 microT(rms). The experimental design respected a double blind procedure and the experimental session involved three steps (pre-testing, exposure, and post-testing). Various measurements were taken, including cognitive performance, psychological parameters such as mood, vigilance, and reporting of symptoms. Physiological parameters such as blood pressure and pulse rate were also recorded. The information given did not significantly modify beliefs. No significant difference was found among the five groups depending on the type of information and the type of exposure in cognitive performance, psychological and physiological parameters. In the context of the study, with our population, the type of information given failed to induce expected changes in parameters measured. Our results do not support the hypothesis that an acute exposure to extremely low frequency magnetic fields (50 Hz, 400 microT(rms)) affects the parameters measured.