1. ;Miniglomera' appearing as small masses of tissue with ample vascularization were found around the common carotid artery of the cat. Physiological, gross anatomical and electron microscopic studies were conducted on these tissues.2. The chemosensory function of each ;miniglomus' was evident from the behaviour of the afferent nerve fibres supplying the tissue: afferent responses became more active during asphyxia, when the blood flow through the tissue was reduced or blocked and when cyanide or ACh were applied. The afferent impulses became more infrequent during hyperventilation.3. Sensory frequency response curves constructed against percentage of inhaled O(2) showed that the impulses of single units increased in frequency with lowering of O(2) content of the inhaled gas.4. These miniglomera are innervated by afferent fibres emerging from the nodose ganglion; sometimes these fibres are contained in the aortic or common carotid baroreceptor nerves, but sometimes they emerge as independent nerves. None of the miniglomera are supplied by branches of the sinus nerve.5. The fine structure of the miniglomus is similar to that of the carotid body. The tissue contains two types of cells: glomus cells which contain dense cored granules, and sustentacular cells whose fine processes enclose the former. Membrane densifications occur where glomus cells lie adjacent to one another or where they are contacted by nerve terminals. Nerve fibres are common in the miniglomus but they contact glomus cells less frequently than in the carotid body.