THE carotid bodies of man and animals are enlarged in states of chronic hypoxia, be it due to high altitude (Arias-Stella, 1969; Edwards et al., 1971) or chronic respiratory disease (Heath, Edwards and Harris, 1970; Edwards, Heath and Harris, 1971). Light microscopy reveals vacuolation of the chief cells of these enlarged carotid bodies and this has been ascribed to autolysis or hypoxia. The present study of the electron microscopy of the carotid bodies from animals at high altitude was undertaken to see whether vacuoles were present immediately after death, thus excluding autolysis, and to determine their ultrastructure and origin within the cell.