The medial preoptic area is a key structure in the neural control of reproduction. Considerable evidence has accumulated indicating that glutamatergic innervation of the area plays an important role in this control. Sources of the glutamatergic input are unknown. Present investigations were aimed at studying this question. [3H]D-aspartate, which is selectively taken up by high-affinity uptake sites at presynaptic endings that use glutamate or aspartate as a transmitter, and is transported back to the cell body, was injected into the medial preoptic area. The neurons retrogradely labelled with [3H]D-aspartate were detected autoradiographically. Labelled cells were found in several telencephalic and diencephalic structures, but not in the brainstem. Within the telencephalon, labelled neurons were detected in the lateral septum, bed nucleus of the stria terminalis and amygdala. Diencephalic structures included the medial preoptic area itself, hypothalamic paraventricular, suprachiasmatic, ventromedial, arcuate, ventral premammillary, supramammillary and thalamic paraventricular nuclei. All of them are known to project to this area. The findings provide the first neuromorphological data on the location of putative glutamatergic neurons projecting to the medial preoptic area. Furthermore, they indicate that local putative glutamatergic neurons as well as several telencephalic and diencephalic structures contribute to the glutamatergic innervation of the area.