The B-adrenoceptor antagonist pindolol, given p.o. (10 mg/kg, once a day for 7 days) to 4 and 24 week old male spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY), decreased plasma angiotensin-converting enzyme (ACE) activity only in young SHR. Kinetic studies indicated that the treatmert ¡educed the ACE maximal velocity or the numbe. of available ACE molecules. In vitro studies showed that pindolol had no direct effects on the activity of plasma ACE. A decrease in soluble (loosely bound) ACE activity was observed in th€ lung of young SHR treated with pindolol. The results suggest that pindolol affects the site of origin of plasma ACE. The susceptibility of young SHR to pindolol may indicate a probable role of B-adrenocepto¡s i¡ the regulation of ACE activily in spontaneous (genetic) hypertension.