AbstractGastrin concentrations were measured in the gastric blood of eviscerated cats. Blood flow was recorded concomitantly thus enabling quantitative determination of the gastrin output. Electrical vagal stimulation within the “physiological” frequency range (0.5–3 Hz) did not regularly result in any detectable changes in the peripheral gastrin level but an increase in this level was always observed on stimulation with frequencies above 3 Hz. A constant gastrin release per stimulus was then obtained, as long as the total number of vagal stimuli did not exceed 2000–3000. A larger number of impulses resulted in rapid fatigue of the release mechanism, indicating either a rapid and sustained fatigue of the neural control mechanism or the existence of a rather small pool of gastrin for immediate release—amounting to less than one per cent of the total antral store of immunoreactive gastrin. The rate at which such a releasable pool was refilled would suffice to compensate for the depletion induced by vagal stimuli at physiological frequencies,i.e.<3–4 Hz only. The two vagi innervate separate gastrin pools, which behave independently as far as release and refilling ar