Although the fine structure of the carotid body has been described in several recent reports, uncertainties remain, and the morphological effects of anoxia on the carotid body cells of the cat have never been reported. We have, therefore, studied the fine structure of the carotid body both in normal and severely anoxic cats, and to test the specificity of the effects, have compared them with the effects on adrenal medulla, kidney, and liver of the same animals. Carotid bodies of 50 normal and 15 severely anoxic cats (9% oxygen in nitrogen) were studied. Glutaraldehyde followed by OsO4 fixations, Epon 812 embedding, and uranyl acetate and lead citrate staining, were the technics employed.We have called the two types of glomus cells enclosed and enclosing cells. They correspond to those previously designated as chemoreceptor and sustentacular cells respectively (1). The enclosed cells forming the vast majority, are irregular in shape with many processes and occasional peripheral densities (Fig. 1).