The aim of this study was to determine if pressor hyper-responsiveness is associated with the hypertension that results from the ingestion of the synthetic estrogen, mestranol. Rats were fed a diet containing mestranol for 6 months, while control rats were fed the same diet without mestranol. Catheters were then placed in the carotid artery and jugular vein of all rats, under halothane anesthesia. When the rats had recovered from the anesthesia, blood pressure was measured through the carotid catheter in conscious rats. Each rat received IV injections of norepinephrine (NE) at 2, 4, 8, and 16 ng per 100 gm body weight, and the pressor responses were recorded. The rats fed mestranol had significantly higher arterial pressures than did the control rats. However, the pressor responses to NE were significantly less in the mestranol-treated rats than in the controls, indicating that pressor hyper-responsiveness does not contribute to this form of hypertension.