The appearance and distribution of zinc-rich terminal fields in the rat forebrain was analyzed at 12 stages of postnatal development using the selenium method. Zinc stain was detected in neonates in piriform, cingulate, and motor cortices, septal area, and hippocampal formation. In the neocortex, a laminar pattern appeared progressively following an inside-out gradient: layer VI at postnatal day 0 (P0), layer V at P1, layers Va and Vb at P5, layer II-III at P9, and layer IV at P12. In the hippocampal formation the layered pattern in the dentate molecular layer appeared at P1-P3, and in the hilus and mossy fibers the stain was observed at P5. Patches in the caudate-putamen were sharply delimited at P1-P3. At these ages, staining was observed in the amygdaloid complex. In the thalamic and hypothalamic nuclei, stain appeared at P5-P7. Thus, a general increase in vesicular zinc over different telencephalic areas was determined until P15-P21, which was followed by a slight decrease thereafter (at P41). The increased stain in zinc-rich terminal fields is consistent with the development of telencephalic circuits. The rise in zinc might be relevant for the establishment and maturation of these circuits. On the other hand, the decrease in staining for zinc at later stages might be due to methodological problems but it might also reflect pruning of supernumerary connections and programmed cell death affecting zinc-rich circuits.