The intrinsic connectivity of striate cortex was investigated by injecting horseradish peroxidase (HRP) into this area in tree shrews. Such HRP injections demonstrated periodically organized, stripelike connections within area 17. These stripes occur in layers I-IIIA and consist of a small number or retrogradely filled neurons, some clearly pyramidal, together with HRP-labeled axon terminals. HRP-filled axons trunks run between labeled stripes, interconnecting adjacent and distant regions of the stripe pattern. Correlation with Golgi-stained tissue suggests that these stripes are horizontally interconnected by pyramidal neurons with long intracortical axon collaterals (followed for distances over 1 mm from the soma). The HRP-labeled strips measure about 230 micrometers in width, with a center-to-center repeat distance of 450--500 micrometers. They have been mapped over an 8 mm2 area of striate cortex and would thus seem capable of effecting lateral interactions over considerable portions of the retinotopic map. In their dimensions and overall pattern, these anatomical stripes resemble the 2-deoxyglucose (2-DG) bands resulting from visual stimulation of trees shrews with stripes of a single orientation. While the functional role of the HRP-labeled stripes is unclear, their similarities with the 2-DG pattern raise the intriguing possibility that they may be related to orientation selectivity. The striking regularity of these extensive lateral interconnections emphasizes the importance of horizontal intralaminar connections within the cortex.