Oxygen tensions in hypothalamus, cerebral cortex, muscle, testis and subcutaneous tissue of anaesthetized rats were measured quantitatively by the oxygen-cathode technique. The resting levels of oxygen tension and the response to breathing oxygen were assessed and then vasodilator or vasoconstrictor drugs were injected and the changes of oxygen tension were observed. 5HT (Serotonin) caused marked falls of oxygen tension in brain, muscle, testis and subcutaneous tissues, lasting 1 to 2 h, when given by intraperitoneal injection (i.p.) and profound but transient falls when given intravenously (i.v.). Noradrenaline (i.v.) caused a rapid transient rise of oxygen tension in brain, a transient fall in muscle, a longer fall in testis, and a variable response in subcutaneous tissue. Reserpine (i.v.) was followed by a slow fall in oxygen tension in brain especially if the body temperature was allowed to fall. (Alone a fall of body temperature tended to increase the oxygen tension in brain.) Amphetamine (i.v. or i.p.) increased the oxygen tension in brain. The mono-amine oxidase inhibitor, tranylcypromine (SKF 385) caused a steady rise of oxygen tension in all tissues studies except testis. The subsequent (i.v.) injection of reserpine caused a rapid, marked increase of oxygen tension in brain. The oxygen tension in muscle fell, in subcutaneous tissue there was a rise followed by a fall which persisted.