Unilateral partial ablations were made in the medial granular frontal cortex of 6 adult squirrel monkeys. Fiber degeneration was traced using the Nauta-Gygax and Fink-Heimer selective silver impregnation techniques into the cingulum, uncinate fascicle, corpus callosum, internal capsule and sublenticular (ventral extracapsular) bundle. Corticocortical projections were observed to the lateral orbital, parietal, rostral inferior temporal, and entorhinal cortices, as well as to the hippocampus proper. Subcortical projections were observed to the basal amygdaloid nucleus, lateral hypothalamus, lateral dorsal, anterior ventral, mediodorsal, pulvinar, and intralaminar nuclei of the thalamus. Preterminal and terminal fiber degeneration were notably sparse in most of the neostriatum, except the dorsolateral region of the caudate nucleus. Fiber degeneration traversed the medial crus cerebri of the midbrain, where fibers of passage ascended into the tegmentum through the medial substantia nigra. Preterminal and terminal degeneration were present in the mesencephalic reticular formation lateral and dorsal to the red nucleus and a few degenerating fibers were followed into the dorsolateral periaqueductal gray. Degeneration was observed as far caudal as the reticulotegmental nucleus of the rostral pons. On the basis of its connections, the medial prefrontal cortex resembles the remainder of the prefrontal lobe and probably also has an important role in cortical and subcortical limbic mechanisms.