Instillation of liver extract into antral pouches produced an increase in the serum concentrations of both little (G-17) and big (G-34) gastrins. The molar fraction of G-17 plus G-34 represented by G-17 was about 0.9 in antral mucosa and about 0.3 in serum 3 hr after initiating release with liver extract. The predominance of G-34 in serum can be accounted for only in part by its slower rate of removal from the blood so other factors probably also contribute. Although G-17 contributed only about 30% of the total molar concentration of gastrins in serum, it accounted for about 70% of the acid stimulatory activity because on a molar basis it is about five times more bioactive than G-34.