In four conscious chair-acclimatized squirrel monkeys (Saimiri sciureus) studied with lights on (600 lx) from 0800 to 2000 h daily (LD 12:12), prominent 24-h rhythms in feeding, drinking, activity, body temperature, and urinary potassium, sodium, and water excretion were seen. When the monkeys were subjected to 36 h of darkness followed by 36 h of light each variable demonstrated a circadian rhythm which was not passively dependent on the light-dark cycle. After the 24-h light-dark cycle was abruptly phase-delayed by 8 h, all the rhythms resynchronized with the new light-dark cycle phase, demonstrating that light-dark cycles are an effective zeitgeber. However, the resynchronization of the rhythms of feeding, drinking, activity, and body temperature was 90% complete within approximately 2 days while the 90% resynchronization of the urinary rhythms took approximately 5 days. These results suggest that the circadian timing system in S. sciureus may consist of several spontaneously oscillating units which can become transiently uncoupled during pertubations of environmental time cues.