Glutamate decarboxylase activity (GAD) was assayed in discrete hypothalamic nuclei in the rat following lesions of the major afferent pathways from hippocampus, amygdala, midbrain, septum, thalamus and globus pallidus. None of these lesions led to a marked decrease in GAD activity in selected nuclei. After total deafferentation of the medial-basal hypothalamus GAD remained unchanged in the median eminence but fell markedly in the ventromedial and arcuate nuclei. In these two nuclei a decrease of GAD still occurred following partial deafferentation from lateral and posterior hypothalamus, but not from anterior and preoptic areas. These results indicated that most GAD-containing cells have their origin inside of the hypothalamus. In this region GABAergic neurons are likely to be short interneurons providing intrahypothalamic connections. Such connections are suggested from lateral and posterior hypothalamus onto the medial basal nuclei.