Morphological and functional data indicate that glutamatergic innervation of the hypothalamic paraventricular nucleus plays an important role in the control of this prominent cell group. Sources of this neural input are unknown. The present investigations were aimed at studying this question. The retrograde tracer [3H]D-aspartate, which is selectively taken up by the terminals of neurons that use glutamate or aspartate as a neurotransmitter, and is retrogradely transported to their perikarya, was injected into the paraventricular nucleus. The brain was examined for labelled neurons visualized by autoradiography. Labelled neurons were detected in the paraventricular nucleus itself, in several hypothalamic areas including medial and lateral preoptic area, suprachiasmatic nucleus, anterior hypothalamic area, ventromedial nucleus, dorsomedial nucleus, lateral hypothalamic area, posterior part of arcuate nucleus, ventral premammillary nucleus and supramammillary nucleus. Outside the hypothalamus labelled neurons were found in the thalamic paraventricular nucleus and in certain telencephalic regions including lateral septum, bed nucleus of the stria terminalis and amygdala. All of them are known to project to the hypothalamic paraventricular nucleus. We failed to detect labelled neurons in the lower brainstem. From these findings we conclude that firstly, there are glutamatergic/aspartatergic interneurons in the paraventricular nucleus; secondly, all intrahypothalamic and telencephalic, but not lower brainstem afferents to this nucleus contain glutamatergic/aspartatergic fibres; and thirdly, the glutamatergic/aspartatergic innervation of this heterogeneous cell group is extremely complex.