The efferent and afferent connections of the supplementary motor area (SMA) were studied in 6 squirrel monkeys using [3H]leucine and horseradish peroxidase, respectively. Efferent projections, common to all leucine-injected animals, were found to the cortical areas 9,8,44,4,2,5,7,24 and 23. Subcortically , efferents were found to the putamen, caudate nucleus, claustrum, the thalamic nuclei reticularis, ventrialis anterior, ventralis lateralis, medialis dorsalis, centralis lateralis, paracentralis , centrum medianum, parafascicularis, centralis superior lateralis, centralis inferior and lateralis posterior, the subthalamic nucleus, field H of Forel, nuel . ruber, reticular formation of midbrain, pons and medulla, the pontine gray and nucl . reticularis tegmenti pontis. Afferent connections exist with the cortical areas 9,8,6,44,4,1,2,5,7, 24 and 23, insula, fronto-parietal operculum and superior temporal sulcus. Subcortical afferent connections exist with the claustrum, nucleus of the diagonal band, nucl . basalis Meynert, basolateral amygdaloid nucleus, the thalamic nuclei ventralis anterior, ventralis lateralis, medialis dorsalis, centralis lateralis, paracentralis , centrum medianum, centralis superior lateralis, centralis inferior, lateralis posterior and pulvinaris , the posterior hypothalamus, ventral tegmental area, nucl . ruber pars parvicellularis , reticular formation of midbrain and pons, locus coeruleus and nucl . centralis superior Bechterew. The projections are discussed with respect to the possible role SMA plays in the voluntary initiation of motor actions.