Six healthy subjects were given a standard breakfast on one occasion and an intravenous infusion of porcine GIP in a dose of 1.0 microgram . kg-1 . h-1 on another. The experiments lasted 90 min. Plasma GIP values were determined with five different porcine GIP antisera, and, depending on the antiserum used, the postinfusion plasma GIP values could be considered both sub- and supra-physiological.