In normotensive Wistar rats (NCR) and spontaneously hypertensive rats (SHR) aging 3-6 d and 5-6, 15-20, 30-40 and 50-70 weeks respectively, position, shape and size of the carotid bodies were studied using ligh-microscopic methods. The anatomical position of the carotid bodies in the SHR was found to be less variable than in the NCR. In the hypertensive animals the glomera carotici were usually situated near the internal carotid artery. The carotid bodies of the NCR were of a more round-oval shape. In all age-groups the carotid bodies were compact and clearly demarked corpuscules. But also outside the principal mass of the glomera carotici small groups of type I cells were found in the surrounding nervous tissue or they appeared as miniglomera or periadventitial type I cells, respectively. These small groups of type I cells were predominantly provable in the 5-6 weeks old animals and more frequently in the NCR than in the SHR. The absolute carotid body volumes increased from birth up to an age of 30-40 weeks, whereas the relative carotid body volume, i.e. when related to the same body mass, became much smaller from birth up to an age of 15-20 weeks in both the normotensive and hypertensive animals. Except for the 5-6 weeks old group the SHR showed significantly greater carotid bodies when compared with the age-matched normotensive rats. In the course of aging in both strains of animals a stronger and more clearly developed interstitial fibrosis of the specific tissue of the carotid bodies occurred.