Abstract Fatigue was measured on 68 air traffic controllers using the following methods: critical fusion frequency (CFF), tapping test, grid tapping test, self-rating. The measurements were taken 9 times within 24 hours over 3 weeks. Stress was measured on the basis of a questionnaire and of catecholamine excretion in urine. The four fatigue tests showed significant agreement. There was a marked decrease in the values after the 6th hour of work. During the night hours, the test values were lower, and the subjects stated they were more tired. For the first work hour and for the 9th to 11th work hour Spearman's correlation coefficients between the two sets of data were calculated. A significant correlation was found (a) between CFF and grid tapping (p<0·02); (b) between CFF and self-rating ‘ refreshed-tired ’ (p<0·05); (c) between grid tapping and self-rating ‘ refreshed-tired ’ (p<0·02). This means that subjects with a marked decrease in CFF or in grid tapping performance have a greater tendency to ‘ tired...