Remarkably large amounts of adenine nucleotides are identified in type L-cells of the carotid body by fluorescence microscopy (labelling with quinacrine) and electron microscopy (uranaffin reaction). At the fine-structural level of the matrix material of specific granules displays enhanced electron density after fixation with uranium ions. It is suggested that ATP is stored within specific granules in addition to catecholamines and proteins. Adenine nucleotides should be considered as one of the secretion products of the chief cells in the carotid body, being capable locally of influencing vascular flow and/or chemoreceptor terminals. Histochemical analysis of the activities leading to a splitting of adenine nucleotides shows a high reactivity with ATP or ADP as substrates. Reaction products are confined to the entire vascular bed of the carotid body. Using AMP or beta-glycerophosphate as substrate, practically no phosphohydrolytic activity could be detected within the carotid body. Thus, the phosphatases are adequate to remove ATP and ADP, but not to form adenosine.