A test battery was constructed on a palmtop computer for ambulatory purposes. This study explored whether the test battery could assess circadian rhythmicity underconstant routine conditions. Performance, body temperature, and subjective sleepiness of 12 healthy subjects were measured. The test battery consisted of a sleepiness questionnaire and three performance tests: a vigilance detection test, a working memory test, and a choice-reaction time test. The subjects were divided into early-start and late-start groups and were subjected to the constant-routine protocol. All tests showed a trough in performance in the early morning around 07:00h and a peak in the evening between 21:00 and 23:00h. In addition, an afternoon decrement in performance was observed between 15:00 and 17:00h. On average, the circadian (peak-to-trough) variation of the performance variables amounted to 16.9% +/- 1.7 SEM of the maximum across subjects. The late starters showed a larger impairment in performance during the morning than the early starters. This could be attributed to prior wakefulness. The characteristics of the performance rhythms found in this study replicate findings in several other studies carried out under constant routine conditions. In conclusion, the present test battery appeared to be a good tool for future assessment of performance under natural conditions.