Experiments were made on 15 mongrel dogs of either sex. Biologically active substances circulating in the blood after administration of noradrenaline (NA) into the left lateral brain ventricle of the dog were detected using the blood bathed organ technique of Vane. Plasma renin activity was also estimated. NA injected intraventricularly (ivc) in doses of 50-500 mug activated the renin--angiotensin system. Phentolamine prolonged the effect of NA. Propranolol abolished this central effect of NA. The increase in the concentration of endogenous NA in the brain by desmethylimipramine and RO 4-1284 also caused an activation of renin--angiotensin system.