Thyrotropin-releasing-hormone (TRH)-synthesizing neurons in the medial and periventricular parvocellular subdivisions of the rat hypothalamic paraventricular nucleus (PVN) are involved in regulation of the anterior pituitary. Since ultrashort feedback regulation of TRH in the hypothalamus has been suggested by physiological studies, we sought to identify the presence of TRH synaptic contacts containing TRH on TRH tuberoinfundibular neurons in the PVN. An immunocytochemical study was performed at light- and electron-microscopic levels using antiserum directed to the N-terminal cryptic sequence of the TRH precursor, preproTRH 25-50. At the light-microscopic level, contacts between TRH immunoreactive (IR) fibers and the perikarya and processes of TRH-IR neurons were observed in medial and periventricular subdivisions of the PVN. At the ultrastructural level, TRH-neurons appeared either tightly juxtaposed to TRH-immunopositive perikarya and dendrites or to establish axodendritic and axosomatic contacts suggestive of synaptic associations. These data provide a morphologic basis to support a neuroendocrine role for TRH or processed forms of proTRH in the PVN and in particular suggest their involvement as neuromodulators in an ultrashort feedback regulation of TRH tuberoinfundibular neurons.