The effect of intravenous administration of a fat emulsion on canine gastric secretion stimulated by intravenous infusion of amino acids, pentagastrin or insulin was studied. The fat preparation was given at a rate of 30 ml/hour for 2 hours, and its effects were compared with those of a comparable amount of saline solution, each given on three separate occasions in each dog. Fat did not alter the Heidenhain pouch secretion stimulated by intravenous amino acids (1.10 versus 1.11 mmol of hydrogen ion, p greater than 0.9) or the gastrostomy secretion stimulated by intravenous insulin (9.22 versus 9.54 mmol of hydrogen ion, p greater than 0.7) but had a modest inhibitory effect on Heidenhain pouch secretion stimulated by pentagastrin (2.75 versus 3.54 mmol of hydrogen ion, p less than 0.05). These data provide indirect support for the contention that the inhibition of gastric secretion by fat in the gut is mediated by the release of an enterogastrone rather than by a direct effect of absorbed fat. Because gastric stimulation by intravenous fat was not observed in the dog, it seems likely that intravenous fat emulsion can be given to seriously ill patients without fear of an increased likelihood of peptic ulceration due to induced gastric hypersecretion.