GABAergic neurons are produced outside cortical proliferative zones and migrate tangentially into the cortex using various routes, including the marginal zone (MZ). Analyzing Nissl and immunohistochemically processed postmortem brain tissue, we observed that between postconceptional week 15 and 20 in human, and between embryonic day 64 and 75 in the cynomolgus monkey, the width of the MZ doubled and the density of small cells expressing calretinin, calbindin, somatostatin and glutamate-decarboxylase (GAD) increased. We also observed that the major sources of these prospective GABAergic neurons migrating through the MZ are parts of the ventricular and subventricular zones characterized by GAD-expressing cells in the following regions: (1) Proliferative zone around the temporal horn of the lateral ventricle is a source of GAD- and somatostatin-expressing migratory cells that enter the MZ through the hippocampal uncus. (2) Septal eminence and part of the medial ganglionic eminence are sources of GAD- and somatostatin-expressing migratory cells that accumulate on the pial surface of the basal telencephalon and then migrate into the MZ. (3) Proliferative zone of the hypothalamus is a source of GAD-expressing cells that migrate to the surface of the basal telencephalon and merge with cells coming from the ganglionic eminence. (4) Proliferative zone of the medial telencephalic wall at the retrosplenial position is a source of GAD-, calretinin- and calbindin-expressing migratory cells entering directly into the hippocampal MZ. These observations suggest that the MZ represents one of the main pathways for GABAergic neuron migration in monkeys and humans during the middle fetal period.