The effects of medium chain triglyceride (MCT), corn oil, and water on serum glucose, ketones, and immunoreactive insulin (IRI) were compared. Fourteen normal subjects ingested 1 gm. per kg. of MCT (composed principally of C8 and CIO fatty acids) and on another occasion an equal volume of water. After water ingestion, glucose, ketones, and IRI did not change appreciably up to five hours. After MCT, a significant rise in serum ketones and IRI was associated with a slight fall in serum glucose. Fourteen normal subjects ingested 1 gm. per kg. corn oil, and on another occasion an equal volume of water. Serum glucose, ketone and IRI concentrations again did not change up to five hours after ingestion of water. After corn oil, no significant rise in ketones occurred. There was a small rise in IRI and a slight fall in glucose. In the eight subjects in whom MCT, corn oil and water were compared a certain degree of insulin secretory response was obtained after MCT and corn oil but not after water ingestion. However, the IRI response after MCT was highest. The rise in IRI without a corresponding change in serum ketones after ingestion of the long chain triglyceride might be attributed to a gastrointestinal β-cytotropic effect. The mechanism of IRI stimulation by MCT also has been related to MCT-induced hyperketonemia and possibly toa direct effect on insulin secretion of small amounts of octanoate in peripheral blood.