We describe a primate model for the study of net cholesterol flux by splanchnic, portal, and hepatic regions concomitant with that of biliary lipid secretion in conscious healthy fed or fasting baboons with exteriorized but physiologically intact enterohepatic circulations. In five fasting baboons studied, we found evidence for low-density lipoprotein (LDL) cholesterol uptake by the liver of 1.0 mg/min, P less than 0.05. Conversely, splanchnic production of LDL cholesterol was found in the fed state (n = 5), amounting to 0.8 mg/min, P less than 0.05. Uptake or secretion of high-density lipoprotein (HDL) cholesterol by either the liver or the intestine could not be detected by transgradient analysis in our animals in either fed or fasting states. Very low-density lipoprotein secretion of 0.4 mg/min by the liver was also found in the fed state. Isotopic biliary cholesterol derived from labeled plasma HDL or LDL cholesterol reflected hepatic LDL cholesterol production in the fed state but LDL cholesterol uptake in the fasting state. Biliary cholesterol secretion amounted to 8.5% of net hepatic lipoprotein cholesterol flux in the fed state and 10% in the fasting state. We conclude that uptake of LDL cholesterol by the liver is appreciable and contributes to biliary sterol secretion in the fasting baboon. Uptake or secretion of HDL cholesterol was not detected in any metabolic beds by transgradient analysis, but in tracer studies appeared to be a major source of lipoprotein cholesterol transferred to bile in the fed state.