A sensitive automated assay was developed for the determination of histamine. The endogenous histamine content of intact rabbit thoracic aorta, and of its separated adventitial and medial layers was determined. The histamine content of the intact aorta was found to be only partially sensitive to compound 48/80. The isolated aorta accumulated labelled histamine against a concentration gradient. The uptake was abolished by cooling to 1 degree, but was unaffected by oxygen lack. The uptake process was sensitive to a variety of agents including extraneuronal catecholamine uptake inhibitors (deoxycorticosterone acetate, normetanephrine) as well as inhibitors of catecholamine and histamine metabolism (tranylcypromine, hydrallazine). The uptake was not affected by amodiaquin, a histamine N-methyl-transferase inhibitor, by the metabolites 1,4-methylhistamine or imidazole acetic acid, by compound 48/80 or by the basic amino acid arginine.