Aortic barorecptor function was studied in spontaneously hypertensive rats (SHR) of various ages and normotensive Wistar rats. The aortic arch was isolated and perfused, and the activity of the left aortic nerve was recorded. The threshold pressure to elicit barerecptor firing was 80-120 mmHg in normotensive Wistar rats. Resetting of barorecptors (threshold pressure 160-180 mmHg) was found in all untreated SHR of 35-70 wk of age. Resetting of barorecptors was prevented in SHR by starting treatment with antihypertensive agents at the age of 11 wk. Treatment of 32-wk old SHR with antihypertensive agents for 4-6 wk resulted in reversal of barorecptor resetting in 50% animals. The percentage of SHR showing complete reversal of resetting did not increase even when the duration of treatment was tripled. In 52- to 64-wk old SHR, treated with antihypertensive agents, reversal of baroceptor resetting was seen in only 30% animals. It was concluded that baroceptor resetting in SHR was secondary to hypertension. Hypertension, in turn, induced hypertrophy of the tunica media of the aorta. Histological studies showed a close correlation between aortic hypertrophy resetting. Aortic hypertrophy may, therefore, be one of the important factors involved in baroceptor resetting.