1. The mechanism of sympathetic nervous inhibition of gastrointestinal movement was investigated in order to determine whether the primary action of sympathetic nerve stimulation was on ganglia or on the smooth muscle itself.2. The effect of sympathetic nerve stimulation did not resemble the effect of ganglionic blockade or that of removing all nervous tone with tetrodotoxin.3. The same frequency of sympathetic nerve stimulation to the rabbit jejunum which inhibited the myogenic spontaneous activity blocked excitatory responses to transmural post-ganglionic cholinergic nerve stimulation and antagonized contractions in response to drugs acting directly on the smooth muscle.4. Acetylcholine output was measured as an indicator of post-ganglionic nervous activity. In contrast to hexamethonium, stimulation of sympathetic nerves at frequencies of stimulation sufficient to relax the muscle did not reduce the output of acetylcholine.5. Vagus nerves and sympathetic nerves to the guinea-pig stomach were stimulated individually and simultaneously in the presence of hyoscine. The relaxant responses to stimulation of the two nerves summated when they were stimulated simultaneously, indicating that the transganglionic vagal pathway was not blocked.6. It is concluded that the effect of sympathetic nerve stimulation is due to the direct action of the released noradrenaline on the smooth muscle.