Abstract (1) The spontaneous and pharmacologically induced release of 3H-DA, synthesized from [ 3H]tyrosine, has been studied in vivo on the cat caudate nucleus. A new technique has been developed that enables to label and superfuse a limited area of the ventricular surface of the caudate nucleus. [ 3H]Tyrosine was applied for a short time (acute labelling) or superfused continuously (continuous labelling). Superfusates obtained after or during the labelling process were collected in serial fractions. (2) 3H-DA released spontaneously in the resting state was identified and estimated quantitatively in collected fractions. (3) Changes in the decline of 3H-DA after acute labelling, observed in anaesthetized animals (N 2O O 2 penthrane), are attributed to an inhibiting effect of the anaesthetic on the release of the newly synthesized 3H-DA. (4) In unanaesthetized cats, marked and rapid increases in the 3H-DA release (5–10 times the resting state level) were induced repetitively by K +, catron and amphetamine during continuous labelling of DA terminals with [ 3H]tyrosine. Both d- and l-amphetamine were potently active when applied topically or injected intravenously. (5) Results obtained in continuous labelling experiments emphasize the importance of the newly synthesized transmitter in release studies.