Intravenous nutrients were infused at 25 and 50% of total base-line daily caloric intake to determine the role of circulating factors on spontaneous food ingestion in young adult male baboons (Papio cynocephalus). Glucose infusion suppressed food intake (15.1%) when 25% of total calories was infused (P less than 0.05) and 41.8% when 50% of total calories was infused (P less than 0.05) for 14-21 days. Both infusions produced basal hyperglycemia (82-172 mg/dl during 25% glucose and 120-239 mg/dl during 50% glucose). Both infusions also caused an increase in circulating insulin (48.1-63.1 microU/ml during 25% glucose and 68.5-77.2 microU/ml during 50% glucose). The simultaneous infusion of exogenous insulin (0.33 mU X kg-1 X min-1) prevented hyperglycemia (85.8-87.9 mg/dl during 25% glucose) but maintained raised basal peripheral insulin levels (52.4-84.4 microU/ml). The 13% suppression of food intake (P less than 0.05) was similar to glucose infusion alone. Comparable infusions of Intralipid as 25 and 50% of total daily calories also suppressed spontaneous food intake but did not produce hyperglycemia or elevated insulin levels. The magnitude of suppression was similar to that of glucose: 16% when 25% of basal calories was infused (P less than 0.05) and 31.3% when 50% of basal calories was infused (P less than 0.05). However, the pattern was different with a more rapid effect, which tended to diminish in time, rather than the slow effect found with glucose, which was maintained for 14 days. We conclude that circulating nutrients can regulate food intake independent of gastrointestinal absorption in primates.(ABSTRACT TRUNCATED AT 250 WORDS)