Application of the Falck & Hillarp histochemical technique to the rabbit carotid body reveals three fluorescent structures: brilliantly fluorescent Type I cells, varicose perivascular nerves, and weakly fluorescent non-varicose fibres. The Type I cell fluorescence is similar to that of a dopamine model system and has the appropriate activation and emission maxima. A catecholamine, identified as dopamine, has been extracted from homogenized carotid bodies, and estimated by the trihydroxyindole procedure. The concentration of the dopamine in the carotid body is estimated to be 20 to 40 $\mu $g/g. This is very much greater than that of the noradrenaline present, of which there is about 1$\cdot $5 $\mu $g/g. The fluorescence of the Type I cells is attributed to the dopamine and it is suggested that the amine may be granule-bound. The unusually high concentration of dopamine could imply that it is not merely a metabolic intermediate in the carotid body.