Relatively few studies on the physiological effects of fat administration have been published. In the present study, whole body and splanchnic oxygen consumption, blood flow, blood temperature, glucose and insulin economy, and arterial and hepatic venous concentrations of hemoglobin, free fatty acids (FFA), and glycerol were measured by indirect calorimetry and catheterization technique in seven healthy men before and during 2.5 h after oral ingestion of 850 kJ of fat and in five healthy men before and during a 2.5-h intravenous (iv) infusion of 850 kJ of fat. Oral fat increased the splanchnic blood flow by 57 ± 25%, reduced the plasma volume by 6 ± 1%, reduced the arterial concentrations of FFA and glycerol by 33 ± 7 and 50 ± 16%, respectively, and increased arterial insulin concentration by 52 ± 12% despite a simultaneous reduction in splanchnic insulin release, thus suggesting a reduction of the extrasplanchnic extraction of insulin. None of these effects occurred during intravenous fat infusion, and it is suggested that intestinal hormones might elicit these effects. Body heat content, unaffected after oral fat, increased by 67 ± 20 kJ during intravenous fat infusion.