The posterior intercostal arteries of six female and four adult male baboons (Papio ursinus) were investigated by latex injection and subsequent dissection to determine their origin and pattern of distribution. Basic morphological and functional similarities with that of man could provide some base-line data pertinent to future experimental studies in the areas of thoracic aortic aneurysm and aortic atherosclerotic lesion in man. Basic similarities were observed between the baboon and man in the number of intersegmental arteries, the origin of the first two pairs of posterior intercostal arteries, and the anastomosis between the superior intercostal and the third posterior intercostal arteries. However, the baboon showed a craniocaudal sequence of paired orifices giving rise to paired arteries; orifices with incompletely divided septum giving rise to paired arteries; single orifices leading to a common trunk, which finally gives rise to paired arteries; and single orifices opening into single arteries. In addition, there was a craniocaudal decrease in the distance between any given pair of intercostal arteries and an increase in the spacing between adjacent pairs of intercostal arteries. Where a single artery supplied one side of the intercostal space, the contralateral side received a collateral branch, either from the preceding or succeeding intercostal artery. Though the basic organisation of the origin and distribution of the posterior intercostal arteries in the baboon is similar to that of man, the differences observed in the baboon might be attributed to factors such as body size, mode of activity, and even the phylogenetic level of development of the baboon.