The chemical transmission in chemosensory apparatus of the carotid labyrinth of toads was studied. In the carotid labyrinth which was perfused using the isolated heart, the stimulation of the cardiac sympathetic nerve remarkably increased chemosensory impulses in the carotid nerve. In view of ineffectiveness of noradrenaline administration, the increase of chemosensory impulses might be attributed neither to the oxygen lack nor to the accumulation of metabolic substances associated with the acceleration of the heart beat, but rather to the effect of a sympathomimetic substance released in the heart. It was found that dopamine was a stimulant substance to the chemosensory apparatus.
The electron microscopic observation of the chemoreceptor of the carotid labyrinth supports the view that the adrenergic transmission ecists in the carotid chemosensory apparatus of the toad.