This study describes the synaptic organization of the thalamic reticular nucleus (TRN) in the rat, cat, and monkey using electron microscopy combined with immunocytochemistry, degeneration, or horseradish peroxidase (HRP) tracing methods. Three morphological types of terminals are described in the TRN of the rat: a small terminal with densely packed spherical vesicles (D-terminal), which originates from the cortex; a large terminal with loosely packed spherical vesicles (L-terminal), which originates in the dorsal thalamus; and a terminal containing flattened synaptic vesicles (F-terminal) that is probably a TRN recurrent collateral. The cortical input to the TRN has been shown by double-labeling studies to terminate directly upon TRN projection neurons. Similar classes of terminals are found in the TRN of cat and monkey, but there is in addition a large terminal with spherical synaptic vesicles that is invaginated by dendritic spines. Also present in the cat and monkey, but not in the rat, are vesicle-containing dendrites and dendritic appendages. In the rat, degeneration experiments indicate that the terminals of TRN projection neurons in the dorsal thalamus are F-terminals. These terminals contain flattened synaptic vesicles and exhibit GABA immunoreactivity.