Three groups of gamma-aminobutyric acid--containing neurons were found in the mammillary region of the posterior hypothalamus. The groups correspond to the tuberal, caudal, and postmammillary caudal magnocellular nuclei. Many cells in these nuclei were retrogradely labeled with fast blue after the injection of this fluorescent dye into the neocortex. Immunohistochemical experiments showed that these same neurons also contained the gamma-aminobutyric acid-synthesizing enzyme glutamate decarboxylase. These results provide morphological evidence for a gamma-aminobutyric acid pathway arising in magnocellular neurons of the posterior hypothalamus and innervating the neocortex.