The laminar distribution and postnatal development of profiles immunoreactive to antibodies directed against serotonin (5-hydroxytryptamine, 5-HT) have been investigated in the primary visual cortex (striate cortex, area 17) of cats. In the adult cat, profiles with serotonin-like immunoreactivity consist exclusively of fibers which exhibit laminar differences in density and predominant orientation. Immunoreactive fibers are dense in layers I-III, less dense in layer V, and sparse in layers IV and VI. In layers I and VI the trajectories of these fibers are mainly tangential to the pial surface; in layers II-V they are predominantly radial and more irregular. The vast majority of immunoreactive fibers consists of fine axons with frequent small varicosities. In addition, there are a few thick axons. In 2-week-old cats, immunoreactive fibers are sparsely distributed through layers II-V. By 4 weeks, fiber density has decreased still further in layer IV and increased in layers I-III. By 6 weeks, the laminar pattern resembles that of adult cats except that fiber density is still lower than in adults. At three months of age, the mature pattern is established.