Zones containing actively dividing cells (proliferation zones: PZs), in the brain of adult three-spined sticklebacks, were identified by autoradiographic detection of (3)H-thymidine and immunocytochemical detection of the thymidine analogue 5'-bromodeoxyuridine (BrdU), singly or in combination, and by immunocytochemical detection of proliferating cell nuclear antigen (PCNA) by monoclonal antibodies. The PZs are associated with boundaries between adult brain regions, as well as with defined morphofunctional subdivisions. PZs are located at the border between the telencephalon and diencephalon, and at the border between the mesencephalon and the rhombencephalon. In the midbrain, the PZ follows the dorsomedial, caudal, and ventrolateral aspects of each tectal hemisphere, extending over the caudal aspect of the torus semicircularis to the nucleus lateralis valvulae. In the hindbrain, the major PZ apparently represents the persisting embryonic secondary matrix layer of the developing cerebellum. In the forebrain, the PZs are associated with the ventricular zones of the olfactory bulbs and ventral telencephalic area ("subpallium"), dorsal telencephalic area ("pallium"), preoptic region, ventral thalamus, dorsal thalamus, epithalamus, pretectum, posterior tuberculum, and the hypothalamus. The diencephalic PZs are parcellated according to a neuromeric organisation (a synencephalic, a posterior, and an anterior parencephalic neuromere: p1, p2, and p3). The PZs of the secondary prosencephalon (telencephalon and hypothalamus) thus would belong to neuromeres p4-6, but do not form an immediately recognised serial pattern. The prosencephalic PZs correlate well with parts of embryonic migration areas as defined by Bergquist and Källén ([1954] J. Comp. Neurol. 100:627-659), morphogenetic fields from which postmitotic neurones migrate to their final destination.