Isoproterenol infusions depress pentagastrin (PG)-stimulated secretion of acid and pepsin from both gastric fistulae and denervated (Heidenhain) pouches in conscious dogs. It was not found to do so if methacholine replaced gastrin. Propranolol reversed the isoproterenol depression of PG stimulation but had no effect on isoproterenol plus methacholine except on the fistula where both acid and pepsin were depressed. It is felt that PG and methacholine act by differing mechanisms both on chief and parietal cells.