Neurotensin-like immunoreactive (NTir) axon terminals in the mediodorsal nucleus of the thalamus (MD) in the adult rat were demonstrated by electron microscopic immunohistochemistry. Most NTir terminals were large (greater than 2 microns in diameter) with round synaptic vesicles and asymmetrical synaptic contacts although smaller (less than 1.5 microns in diameter) axon terminals were also labeled. Both types of terminals were found in the medial and central parts of MD with the greatest density in the medial part. These NTir boutons have similar ultrastructural features as anterogradely labeled terminals from the piriform cortex and the preoptic area, which have previously been identified as sources of NTir axons in MD. A few NTir boutons were also found in the medial part of MD with pleomorphic vesicles and symmetrical synaptic contacts.