The carotid bodies of F1- and F2-hybrids of spontaneously hypertensive rats of the Okamoto-Aoki strain (SHR) and normotensive Wistar rats (NWR) were studied. Both the F1- and the F2-generation exhibited an elevated mean arterial blood pressure. In the F1-hybrids smaller carotid bodies and fewer intraglomic vascular changes, but higher mean arterial blood pressure than in the F2-hybrids were found. In both groups there was no correlation between carotid body size and the occurrence of intraglomic vessel alterations. In general, vascular changes were less developed than in age-matched SHR. It is concluded that in SHR/NWR F1- and F2-hybrids the development of carotid body enlargement is not only dependent on an elevated blood pressure or the presence of intraglomic pad-like vascular alterations.