Serotonergic innervation formed transient dense aggregation in all rat primary sensory areas during early postnatal development. In the normal rat visual cortex, there were three stages in the transient aggregation of serotonin immunoreactive fibers from postnatal day (PND) 2 to 15. Primary aggregates appeared in the subplate prior to the differentiation of layer IV, and moved upward to the lower layer of cortical plate to form secondary aggregates. Secondary aggregation gave rise to ascending fibers on the cortical surface which were roughly grouped in columns (tertiary aggregation). The serotonin fibers of the tertiary aggregates were arranged in a lattice-like pattern in layer I, and were significantly altered either in size of distribution area or column number after monocular or binocular enucleation. These data indicate that the lattice-like pattern of serotonin innervation in neonatal rats may be an evolutionary precursor of the "blobs" seen in the higher primates.