Spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto rats (WKY), 25 weeks of age, were treated for 23 weeks with hydrochlorothiazide, hydralazine, and reserpine added to their drinking water. Cylindrical segments of mesenteric arteries (about 170 µm diameter, and 0.7 mm long) were isolated and mounted in a myograph for dimensional and circumferential tension measurements. Treatment of the SHR and WKY decreased systolic blood pressures by 29% and the smooth muscle cell (SMC) content of these vessels by 31%. A linear relation was found between blood pressure and SMC content for all treated and untreated rats. Our results suggest that effect of treatment on the cellular content of the media was sufficient to account entirely for the maintenance of the lowered blood pressure.