1. Intravenous infusion of propranolol (3·86 μmol h—1 kg—1) for 2 h lowered arterial pressure in the conscious rabbit whereas similar infusions of practolol (37·6 μmol h—1 kg—1) did not.
2. The fall in blood pressure produced by propranolol was accompanied by a decrease in splanchnic nerve activity. Practolol did not change splanchnic nerve activity.
3. A centrally mediated decrease in sympathetic nervous activity makes an important contribution to the hypotensive action of propranolol in the rabbit. This effect is not shown by practolol.