The increased growth potential of vascular smooth muscle cells (VSMCs) represents one of the crucial anomalies responsible for the development of essential hypertension, diabetic macroangiopathy, and atherosclerosis. The exaggerated response to growth factors of VSMC from spontaneously hypertensive rats (SHRs) persists in culture when compared with normotensive Wistar-Kyoto control rats, indicating an intrinsic defect in the hypertension-producing mechanism. This greater proliferation is characterized by two intermediate phenotypes: (1) accelerated entry into the S phase of the cell cycle, which results from hyperresponsiveness to epidermal growth factor and platelet-derived growth factor, and (2) abnormal contact inhibition. The enhanced expression of transforming growth factor beta 1 (TGF-beta 1) messenger ribonucleic acid in SHRs precedes this altered contact inhibition, and only VSMCs from SHRs respond to exogenously added TGF-beta 1 at a high cell density, which suggests that abnormal TGF-beta 1 autoregulation may be implicated in the second phenotype. Platelets contain major growth factors for VSMC. Platelet extracts from hypertensive and diabetic patients present augmented growth-promoting activity on VSMCs, which is most evident when both diseases occur simultaneously. Growth-promoting activity may be further influenced by antihypertensive therapy. This growth-promoting activity is increased by hydrochlorothiazide but not by indapamide, atenolol, or captopril in diabetic hypertensive and nondiabetic hypertensive patients. In conclusion, VSMCs in hypertension manifest an intrinsic growth defect that is modulated by extrinsic platelet growth factors and antihypertensive drugs.