14C-angiotensin II (14C-AII) was infused into rats sacrificed 5, 15, 30 and 60 min post-infusion. Blood and brain samples were extracted and chromatographed on paper with standards of authentic 14C-AII and metabolites produced by the enzymatic degradation of the 14C-AII. Blood levels of both total radioactivity and 14C-angiotensin declined rapidly at 1st and slowly thereafter. Similarly total brain radioactivity also declined with time but 14C-angiotensin levels increased over the 60 min resulting in a large increase in the percentage of angiotensin found in the brain. Examination of the brain/blood ratio for 14C-angiotensin at each time point indicates that the brain reaches equilibrium with the blood (brain/blood = 1) at 30 min and remains unchanged at 60 min. 14C-Angiotensin II i.v. injected can cross the blood brain barrier by a passive mechanism resulting in the binding of the hormone at central sites.