A specific antibody raised against 5-hydroxytryptamine (5-HT) conjugated to bovine serum albumin was used to study the serotoninergic innervation of the basal ganglia in the squirrel monkey (Saimiri sciureus). At midbrain level, numerous fine 5-HT-immunoreactive axons were seen to arise from the immunopositive neurons of the dorsal raphe nucleus and less abundantly from those of the nucleus centralis superior. The bulk of these axons formed a rather loosely arranged bundle that arched ventrorostrally through the central portion of the midbrain tegmentum and ascended toward the ventral tegmental area. Several fascicles detached themselves from this bundle to reach the substantia nigra where they arborized into a multitude of heterogeneously distributed 5-HT terminals. The 5-HT innervation was particularly dense in the pars reticulata but much less so in the pars compacta of the substantia nigra. More rostrally other 5-HT fibers swept dorsolaterally and formed a remarkably dense network of varicose fibers within the subthalamic nucleus. A multitude of 5-HT axons continued their ascending course within the lateral hypothalamic area, and many of them swept laterally to invade the lenticular nucleus. At pallidal levels, the 5-HT axons arborized much less profusely in the external segment than in the internal segment, which contained numerous 5-HT varicose fibers and terminals arranged in a typical bandlike pattern. At striatal levels, the 5-HT terminals were particularly abundant in the ventral striatum, including the nucleus accumbens and deep layers of the olfactory tubercle. They also abounded in the ventrolateral region of the putamen and the ventromedial aspect of the caudate nucleus. Overall, the number of 5-HT fibers and terminals decreased progressively along the rostrocaudal axis of the striatum and several large and elongated zones rather devoid of 5-HT immunoreactivity were visualized, particularly in the caudate nucleus and the dorsal putamen. These zones of poor 5-HT immunoreactivity were in register with similar areas devoid of tyrosine hydroxylase immunoreactivity as seen on contiguous sections. These findings reveal that all the core structures of the basal ganglia in primates receive a significant serotoninergic input, but that the densities and patterns of innervation vary markedly from one structure to the other.