The presence of monoamines in the carotid body cells of the cat was investigated using histochemical methods, microspectrofluorometry and chemical analyses. Large amounts of norepinephrine (31 to 51 ng/carotid body), dopamine (122 to 131 ng/carotid body) and DOPA (86 to 150 ng/carotid body) were found in pooled tissues analyzed with specific chemical techniques. The majority of the glomus cells exhibited an intense fluorescence after formaldehyde treatment, having a spectrum characteristic of catecholamines. Some, but not all, of these cells gave a fluorescence reaction with the trihydroxyindole histochemical method. It is concluded that most of the glomus cells contain norepinephrine or DOPA or a combination of these two compounds while some cells contain only dopamine. A few yellow fluorescent cells were also found after formaldehyde treatment which probably contain 5-hydroxytryptamine.