1. Rectal temperature, electrolyte excretion and performance were studied in young adults who followed an irregular pattern of work and rest for 9 days in an isolation unit. 2. In the analysis, effects evoked by the pattern of work and rest were separated from the oscillatory component, and rhythms for individual days were examined by the cosinor method. 3. During the schedule, rhythms no longer showed a period of exactly 24 h, and this effect was confirmed by studies using a repeated cycle of irregular work and rest and by studies using constant routines. 4. Temperature and urinary constituents differed in the strength and phase of their rhythms when corrected for evoked effects, as well as in the strength of the evoked effects themselves. 5. There was evidence of deterioration in performance during work periods which exceeded 9 h, but there was no evidence of progressive deterioration in performance over the 9 day schedule.