Isolated nerves of cats and frogs were stimulated in vitro in Ringer soln. containing physostigmine (1:100,000), or in simple Ringer. After dialysis the solns. were tested on the isolated frog heart. Parasympathetic nerves and ganglia, and sympathetic pre-ganglionic nerves and ganglia liberated acetylcholine in different amounts. Nerve trunks composed only of ad-renergic fibers and preganglionically denervated superior cervical ganglia, when stimulated, liberated no acetylcholine but an adrenaline-like substance. Mixed nerves, such as frog and cat sciatics, and innervated sympathetic ganglia, released, when stimulated, both acetylcholine and adrenaline. This release occurred in satisfactory physiological conditions [long dash]i.e., the nerves were not damaged by the procedures followed. Cholinergic nerve fibers evidently liberated acetylcholine and adrenergic fibers liberated adrenaline during stimulation.