The physiological rhythms of subjects living on a day of abnormal length.

1. Fourteen subjects, singly or in groups, have been observed while living on a 21 hr day for 8 or 16 experimental 'days' and fifteen other subjects similarly on a 27 hr day. 2. Rhythmic components of body temperature and excretion of various urinary constituents were calculated. 3. On a 21 hr day, for most components and most subjects, two periods were present, one of 21 hr and one of around or somewhat over 24 hr. 4. On a 27 hr day two periods were less often present and a larger number of observed rhythms could be satisfactorily described by a single period, usually between 23 and 28 hr. 5. In subjects spending a second week on a 21 hr day the circadian component was no less prominent than during the first week. 6. When, after life on a 21 hr day, subjects were deprived of knowledge of time, there was evidence that the 21 hr component did not persist. 7. The results are interpreted as evidence of the continuing existance of an influence with a period of around 24 hr, simultaneously rhythmic influences resulting from the subjects' habits. On a 27 hr day there was sometimes evidence of entrainment, yielding an intermediate period. 8. An attempt is made to compare the relative potency of the exogenous and of the persistent circadian influences on the several variables.