When 3H-proline was administered to spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats, its incorporation into non-collagen protein of the posterior cerebral artery or the mesenteric artery was greater in SHR than WKY. Young SHR were treated with either clonidine or 6-hydroxydopamine (6-OHDA). Administration of clonidine decreased the rate of incorporation of 3H-proline into the non-collagen protein of these arteries concomitantly with a reduction of blood pressure in each rat strain. Administration of 6-OHDA slightly decreased the incorporation of the proline into the non-collagen protein of the SHR posterior cerebral artery, but failed to reduce blood pressure in each rat strain. Ultrastructural pictures of the posterior cerebral arteries of the SHR showed numerous adrenergic ganglion cells. Thus, neural factors, especially in the cerebral arteries, may regulate vascular protein synthesis and may be important for the initiation of hypertension in SHR.