Three synthetic preparations of big gastrin (G-34), little gastrin (G-17) and minigastrin (G-14) have been compared with regard to gastric acid stimulatory potency, rate of disappearance and the relation between acid secretion and the change in serum immunoreactive gastrin in gastric fistula and Heidenhain pouch dogs. Equimolar graded doses (12.5 to 200 pmol/kg-hr) of G-14, G-17 and G-34 produced similar rates of acid secretion with equal ED50 for all three gastrin peptides. Equimolar doses of G-14 and G-17 also resulted in similar increments in serum immunoreactive gastrin, but those of G-34 caused approximately two to three times greater increments in serum gastrin than did G-14 or G-17. The disappearance half-times determined by measuring serum gastrin at short intervals after discontinuation of equimolar dose (1000 pmol/kg-hr) infusions of each gastrin peptide were 1.75, 4.85 and 11.53 min for G-14, G-17 and G-34, respectively. The calculated space of distribution was similar for all three gastrin preparations and ranged from 20-30% body weight. These results indicate that the three major gastrin components differ in half-times and in relating secretory potency, in that relatively higher molar concentrations of G-34 than G-14 or G-17 were required to produce equal rates of acid secretion.