In this study we used bromodeoxyuridine (BrdU), a thymidine analogue that is incorporated into the DNA of mitotic cells, to study the cytogenesis status of the striatum in normal, adult, squirrel monkeys (Saimiri sciureus). Three weeks following BrdU injection, numerous BrdU-labeled (+) cells were encountered within both the dorsal and the ventral striatum, including the nucleus accumbens. Their number ranged from 5 to 50 per 40 microm-thick section. These BrdU+ cells were more abundant medially than laterally and displayed a rostrocaudal-decreasing gradient in the caudate nucleus and putamen. Double-immunofluorescence confocal studies have revealed that about 5-10% of the BrdU+ striatal cells expressed the neuronal nuclear antigen (NeuN), a marker for mature neurons. These findings suggest that new neurons are produced throughout adult life in the striatum of normal, adult primates. This result raises the possibility of experimentally enhancing the recruitment of these newborn neurons as a means to alleviate the symptoms of neurodegenerative diseases that affect the striatum.