This research examines the possible fatigue induced in workers submitted to a compressed 4-40 work-week (i.e. 40 h in 4 days, as compared to the usual 5-40 schedule, i.e. 40 h in 5 days). Two groups of workers from two different factories (4-40 and 5-40 work schedules) manufacturing similar products participated in the study. The analysis of data from measurements of reaction time, heart rate, blood pressure, body temperature, respiratory volumes, O2 consumption, CO2 production etc., did not reveal any significant differences between the two groups measured before and after the first and the last day of the work-week. However, the critical flicker fusion frequency and the right hand strength showed a significant greater impairment in the 4-40 workers. We conclude that The fatigue originating from the tasks accomplished by these 4-40 workers is as considerable as the one encountered by the workers of a similar factory working under a 5-40 schedule.