This study compares the basal ganglia of rats, marmosets, macaques, baboons, and humans. It uses established protocols to estimate the volume and number of neurons within the output nuclei (internal globus pallidus, IGP; and nondopaminergic substantia nigra, SNND), two internal relay and modulating nuclei (subthalamic nucleus, STh; and external globus pallidus, EGP), and a modulator of the striatum (dopaminergic substantia nigra, SND). Nuclear boundaries were defined by using immunohistochemistry for striatal afferents. Total numbers of Nissl-stained and parvalbumin-immunoreactive neurons were calculated by using the fractionator technique. Comparisons between species were standardized relative to brain mass (rats < marmosets < macaques < baboons < humans). The EGP consistently had more neurons relative to the IGP, STh, and SND, which had similar neuronal numbers within each species. The SNND had proportionally more neurons in rats than in primates (especially humans). The distribution of SND neurons varied substantially between rats and primates (very few ventrally located neurons in rats) with humans containing fewer SND neurons than other primates. The reduction in SND neurons in humans suggests less dopaminergic regulation of the basal ganglia system compared with other species. The consistency in the number of IGP neurons across all species, combined with the reduction in SNND neurons in humans, suggests a greater emphasis on output pathways through the IGP and that there are proportionally more STh and EGP neurons in humans. J. Comp. Neurol. 445:238–255, 2002. © 2002 Wiley-Liss, Inc.