Tne density of adrenergic nerves at various levels in the guinea-pig lung.

1. Gunea-pig tracheal and lung segments were systematically examined for adrenergic nerves using the fluorescence histochemical technique. Fluorescence in nerves was enhanced with alpha-methylnoradrenaline or by pretreating animals with nialamide. 2. There was a progressive decrese in density of fluorescent fibres in airway smooth muscle from the laryngeal end of the trachea, which was densely innervated, to the bronchioles, which contained only occasional fibres. This was despite obvious fluorescence in blood vessels in the lung. 3. It is suggested that, if adrenergic nerves are involved in reflex bronchodilatation in this species, they directly control larger airways and other factors, e.g. non-adrenergic inhibitory nerves, may be important in the control of smaller airways.