A study of the comparative effects of a 10 hour (hr) and 8hr work schedule on mental performance and alertness in air traffic controllers (ATCs) was conducted using the NIOSH Fatigue Test Battery. Fifty two ATCs, 86% of whom were male, 28 to 50 years old, participated in the study. Twenty six were working on a traditional 8hr 2/2/1 rapidly rotating shift schedule and 26 were working on a 10hr 4 day rotating shift schedule. The subjects working on the 10hr shift system were approximately 4 years older than those on the 8hr schedule. The subjects were evaluated on the NIOSH Fatigue Test Battery three times daily over a 3 week period: at the start of their shift, 2hr before the end of their shift, and at the end of the shift. The NIOSH battery was a self administered computerized program which measured the performance of the subjects on a choice reaction task (CRT) and digit addition and grammatical reasoning tests. The program also collected information on sleep quality, mood, and somatic complaints. No significant difference in performance on any of the tests were seen between those working on the 10 and 8hr shift schedules during the first 4 days of the work week. Performance on the CRT and digit addition test deteriorated significantly in subjects working on the 8hr shift system on the final day of the five day work week. Some evidence of within and cross/shift changes in alertness, as indicated by changes in performance on the tests, were seen in workers on both shift systems. For example, their average reaction times on the CRT were significantly faster on the first two days than in day 3 or 4. Mood ratings and perceived sleep quality decreased as the work week progressed. The decreases were more pronounced in workers on the 8hr shift system. Very few somatic complaints were reported by any of the subjects and there was no evidence of any significant changes in somatic complaints across the work week. The authors conclude that the results suggest the 10hr shift schedule is acceptable; however, none of the ATCs on the 10hr schedule had a night shift during the study. The requirements to provide 24hr ATC coverage and to have adequate staffing across the workday, however, require that various 8hr shift schedules be continued. The results of this study do present one example of an approach that can be used to develop a database for making decisions about the effects of rapidly rotating (8hr) shift schedules versus compressed (10hr) shift schedules.