Abstract--We investigated the time relationship between changes in blood pressure and changes in the structure of the resistance vasculature. Blood pressure, heart/body weight ratio, and morphology and function of mesenteric resistance arteries from 1-kidney, 1-clip renovascular hypertensive rats were followed before and after declipping at age 14 weeks. The rats were divided into 5 groups, which were investigated 6 hours, 24 hours, 1 week, 4 weeks, and 8 weeks after declipping and compared with 2 normotensive and 2 renovascular hypertensive control groups at 14 weeks and 18 weeks. Systolic blood pressure was elevated 2 weeks after application of the clip and stabilized after 6 weeks. Declipping induced a prompt fall in blood pressure within 6 hours, and blood pressure was normalized within 1 week. Heart/body weight ratio was increased in renovascular hypertensive rats, and declipping induced a gradual decrease in the ratio, which was normalized within 4 weeks. Media/lumen ratio and media area of mesenteric resistance arteries were increased in renovascular hypertensive rats, and declipping did not affect media/lumen ratio and media area within 8 weeks, although there was a tendency for some regression of media/lumen ratio. There were no differences in response to high potassium, noradrenaline, or acetylcholine. Thus, these findings show definitively that declipping causes rapid reversal of renovascular hypertension in rats accompanied by gradual reduction of the heart/body weight ratio but lack of normalization in the mesenteric resistance vessels. This provides clear evidence that neither vascular nor cardiac structural changes are capable of keeping rats hypertensive.