To investigate the mechanism of circadian synchronization by oral food consumption, we tested the effectiveness of cyclic intragastric feeding in synchronizing squirrel monkeys to a 24-hour schedule. Five monkeys were prepared with intragastric catheters and were studied free-ranging within a cage in an environmentally controlled chamber with continuous monitoring of body temperature (every 15 min) and locomotor activity (every 30 min). A balanced mixture of dextrose, amino acids, fat emulsion, vitamins and minerals was infused from 0800-2000 every day via the gastrostomy, first during a 12-hour light and 12-hour dark cycle (lights on 0800-2000 hours), and then subsequently in constant illumination. Serial phase analysis was performed by computing cycle-by-cycle acrophases of best-fitting sinusoids. One animal was synchronized by the feeding regimen while four others exhibited relative coordination. We conclude from these results that the timing of intragastic feeding may have only a weak synchronizing effect on primate circadian rhythms and that pre-gastric cues associated with oral food ingestion contribute to entrainment by feeding schedules.