1. The purpose of this study was to examine the contribution of the sympatho-adrenomedullary system to the blood pressure response to an intravenous bolus of thyrotropin-releasing hormone (TRH) in conscious medullectomized and sham-operated rats. 2. The peak presser effect of 0.5 mg TRH was significantly increased in rats having no adrenal medulla (+24.2+/-1.6 mmHg, mean+/-s.e.m., P<0.01) as compared to sham-operated animals (+12.2+/-3.0 mmHg).