By means of transumbilical portal catheterization immunoreactive insulin (IRI) concentrations in portal vein (POV) and in peripheral vein (PEV) were compared after administration of glucose, galactose, xylitol and tolbutamide. Fasting portal IRI concentration was about twice that found in PEV. After β-cell-stimulation the maximal IRI rise in POV was several times greater compared to PEV. Xylitol produced a significant IRI rise in POV but non in PEV. Thus, absence of IRI elevation in PEV does not exclude stimulation of IRI secretion. Factors affecting the difference between portal and peripheral venous IRI concentrations are discussed. 1 These studies were supported in part by a grant from the Schweiz. Nationalfonds (348670). - Parts of the results of this work have been presented at the 7th Congress of the International Diabetes Federation, Buenos Aires 1970.