Distribution of somatostatin-immunoreactive cells in the cerebral cortex of macaque monkeys at embryonic day 120 (E120), E140, newborn, postnatal day 60 (P60) and adult stages were studied by the avidin-biotin-peroxidase immunohistochemical method. At all stages, there existed 3 types of cells in the gray matter: bipolar, multipolar and small-sized cells which stained only in perikaryon. Somatostatin-immunoreactive cells were observed from E120. The cell number increased between E120 and E140 and decreased until P60. At the newborn stage, a high density of cells was distributed in layer II of the prefrontal and parietal cortices (areas FD and PE). In layer I of the postcentral, parietal, temporal and preoccipital cortices (areas FA, PC, PE, TA, TE and OA), small numbers of horizontal cells were detected only at the embryonic and newborn stages. In adulthood, the number of somatostatin cells was much smaller than at the early stages (E140 and newborn). Compared to other cortical areas, in occipital cortex (area OC), there was little change in cell number during development. In occipito-temporal cortices, there were increases in cell number from posterior to anterior portion at all the stages. The large number of somatostatin cells in all layers of the cerebral cortex during the early stages indicates that somatostatin plays a role in the development of the monkey cerebral cortex.