The circadian rhythms of most shiftworkers do not adapt to night shift. We have studied oil workers on a rotating system involving 2 weeks day shift (0600-1800 h) and 2 weeks night shift (1800-0600 h) throughout a day and night shift sequence. Urine samples were collected 3-hourly whilst awake, with an over-sleep collection, for the measurement of 6-sulphatoxymelatonin by radioimmunoassay. In three separate groups results showed adaptation by delay of the 6-sulphatoxymelatonin rhythm in the first week of night shift. The rates of phase shift (mean +/- SEM) were 1.51 +/- 0.16 h/day (n = 5), 1.32 +/- 0.41 h/day (n = 5) and 1.77 +/- 0.31 h/day (n = 17). Specific environmental and social factors together with the shift schedule on oil rigs may facilitate adaptation to a 12 h night shift within a week.