The transient aggregation of serotonin (5-HT)-containing fibers in the early development of rat visual cortex was examined immunohistochemically. The aggregation of 5-HT immunoreactive (IR) fibers consisted of three stages which were classified according to the course of time and degree of space occupied. The primary aggregation appeared in the subplate and moved upward along the development of the cortex. The aggregation proceeded to the secondary stage in presumptive layer IV. The fibers extended in a column-like structure following the secondary aggregation and formed the tertiary aggregation. The upper edge of the tertiary aggregation formed a lattice-like pattern in layer I and its structure was recognized to be similar to the structure of a 'blob' which characterizes the primary visual cortex in monkey. This transient aggregation of 5-HT-IR fibers began in the subplate of the anterior visual cortex on postnatal day 2 (PND 2) and progressed towards the posterior. On PND 11, the secondary and tertiary aggregations were completed in the entire region. No further aggregation of 5-HT-IR fibers was observed on PND 15. The anterior-to-posterior axis in the aggregation process corresponds to the direction of differentiation in the layer structure of cortex. In order to investigate the relationship between the transient aggregation of 5-HT-IR fibers and the development of the visual pathway, the secondary and tertiary aggregation on PND 11 were observed after postnatal monocular or binocular enucleation. Enucleation of eye balls did not affect either the area occupied by the 5-HT-IR fibers in the secondary aggregation or the number of column structures in the tertiary aggregation. However, the contralateral and ipsilateral cortices of monocularly enucleated cases were irregularly shaped in the secondary aggregation. The distribution of 5-HT-IR fiber terminals in the binocular area (Oc1B) increased in density on the contralateral side in the monocular enucleation, while that of both sides in the binocular enucleation was of non-homogeneous density and were shaped irregularly. The above results suggest that the transient aggregation of 5-HT-IR fibers observed in the early stage of development of visual cortex is regulated primarily by the intrinsic factors, and that extrinsic factors, such as visual pathway input, affect the aggregation within the boundary of such intrinsic factors. That is, the visual pathway input and the input balance from both eyes affect the distribution density of 5-HT-IR fibers and the shape of the visual cortex, respectively.