The influence of a chronic blood pressure lowering on carotid body morphology and volume was studied in spontaneously hypertensive rats (SHR). Oral propranolol, a beta-adrenergic blocking agent, was administered beginning 3 weeks after conception up to 18 weeks of the litters. At the end of the experiment the blood pressure was significantly lower in the treated animals. These rats exhibited fewer vascular alterations in their carotid bodies. The carotid body volumes and the relationship of glomic cells, the blood vessels including endothelial cells, and the connective tissue in the carotid bodies did not differ between treated and non-treated rats. It can be concluded that long-term blood pressure lowering by propranolol in SHR does not influence carotid body volume increase.