Pyramidal cells in layer 2/3 of cat striate cortex extend long axons that form clustered projections linking iso-orientation columns. Using extracellular biocytin injections in brain slices, the formation of these projections was examined in the ferret to determine whether horizontal projections exhibit similar patterns of development in the ferret and the cat, and to relate the time course of horizontal projection formation to the onset of patterned visual experience and orientation selectivity. Soon after the first appearance of axon collaterals in layer 2/3, around postnatal day 22 (P22), pyramidal cell axons were uniformly distributed and unbranched for up to 1 mm from the cell body. By P26, axons began to form secondary branches 1-2 mm from the cell body, with little evidence for distinct clusters. The first indication of selective elaboration of secondary branches and retraction of unbranched collaterals occurred around P28. By P34, patchy regions of axon branches emerged, though unbranched collaterals were still present, followed by distinct, adult-like clusters by P45. Although the general pattern of horizontal projection formation closely resembles that seen in the cat (Callaway and Katz, 1990), the ferret circuitry matures earlier than that of the cat relative to the time of eye opening. Since eye opening in ferrets occurs between P30 and P32, this system of orientation-specific patches begins to develop in the absence of patterned visual input and when most cortical cells are not yet orientation selective, suggesting a prominent role for spontaneous activity in initiating cluster formation. The refinement of clustered connections, however, does occur synchronously with the maturation of orientation-selective responses (Chapman and Stryker, 1993).