Cold exposure of rats for 4 h and simultaneous inhibition of dopamine beta-hydroxylase by FLA-63 (25mg/kg) led to a reduction of the catecholamine content of the adrenal medulla by 46% and of the brain by 68%. Additional injections of 5 mg/kg FLA-63 4 and 9 h after beginning of the experiments, respectively, kept the catecholamine content on this low level (brain) or decreased it further (adrenal medulla). Administration of 5 mg/kg (-)DOPA together with the mono-amine oxidase inhibitor pargyline (50 mg/kg) 24 h after the first injection of FLA-63 stimulated the resynthesis. It amounted for the adrenal medulla to 20 mug/kg body weight/8 h and for the brain to 45ng/g tissue wet weight/8 h. Paper chromatographic analyses of the extracts of adrenal medulla and brain, respectively, performed at each time of the different injections, clearly identified adrenaline, noradrenaline and dopamine (in traces) in the adrenal medulla as well as noradrenaline and dopamine in the brain; epinine on the contray could not be demonstrated, not even in traces. Since at least 25 ng of epinine can be detected with certainty by our method, it can be concluded that epinine is not formed in amounts greater than 75 ng/pair adrenal glands or 37.5 ng/brain. The present results support the view that the main pathway of adrenaline biosynthesis in the suprarenal medulla and the brain proceeds via noradrenaline and not via epinine.