The aim of the study was to analyse the effects of rapid time zone changes on the circadian rhythms of flight attendants. The mean age of the 40 female subjects was 30.0 (SD = 6.9) years. Measurements of oral temperature, alertness, and visual search were performed at two hour intervals two days before the flight from Helsinki to Los Angeles, during the second and the fourth day in the USA and during the second and fourth day after the return flight to Finland. The body temperature desynchronized and the phases of the alertness and visual search rhythms shifted rapidly in the USA. After the return flight, the acrophases of the circadian rhythms delayed during the second and fourth day in Finland. During the fourth day the acrophase of alertness was 35 min and the acrophases of body temperature and visual search were 2 h 2 min and 3 h 8 min delayed, respectively. The mathematical model based on the C-, S- and W-process theory of alertness explained 25-96% of the variation of observed mean alertness of the subjects in different conditions. It is concluded that the duration of the de- and resynchronization process of the flight attendants' circadian rhythms is on the average longer than 9 days during and after round flights over ten time zones. The mean alertness of the subject can be predicted with considerable accuracy using the mathematical model.