Pentagastrin (0.2–6.4 μg/kg s.c.) given to conscious dogs supplied with a gastric fistula, caused a log dose-dependent gastric secretion of volume, H+, Na+, K+, Ca++, Mg++ and Cl-. The pepsin output was at a similar level as after histamine, but much lower than after insulin-induced vagal stimulation. Atropine methyl nitrate (5 μg/kg i.v.) diminished the secretory response to pentagastrin; dihydroergotamine (40 μg/kg i.v.) augmented the pentagastrin-induced secretion. The significance of pepsin as parameter of the relative tone in the parasympathetic as well as in the sympathetic nervous system is emphasized.