Neurotensin (NT) and dopamine are two neurotransmitters which are present in the hypothalamus of mammals and are often distributed in identical areas. In particular, in the periventricular anterior hypothalamus and in the arcuate nucleus, images of apposition between perikarya and fibers containing dopamine or neurotensin have frequently been observed at the light microscope level. The aim of this study was to answer, at the ultrastructural level in the A12 and A14 catecholaminergic cell groups, the question as to the existence of the possible synaptic nature of such contacts. To this end, NT and tyrosine hydroxylase (TH) were simultaneously visualized using double pre-embedding immunocytochemical methods. In the A12 arcuate area, synaptic contacts were demonstrated between TH-immunoreactive terminals and NT-labeled perikarya and dendrites. The opposite pattern, i.e., NT-stained terminals synapsing onto TH-positive neurons, was also observed. In contrast, only NT synaptic inputs onto TH-stained cell bodies could be demonstrated in the hypothalamic periventricular nucleus. In addition, immunoreactive terminals stained for NT or TH were observed to make synaptic contacts with perikaryal profiles stained for the same antigen. These results demonstrate a strong synaptic NT input onto the dopaminergic neurons of the mediobasal hypothalamus and suggest a reciprocal influence, at least in part, of catecholaminergic terminals on arcuate NT-containing neurons.