The synaptic organization of the subthalamic nucleus (Sth) of the cat has been investigated by means of electron microscopy. On the basis of the following criteria: the size and the shape of the synaptic boutons, their origin, the size and the shape of the synaptic vesicles, the distribution and density of the vesicular population, and the characteristics of the active synaptic zones, several types of synaptic boutons have been discriminated: F1, F2, SR, LR1, LR2, d.c.v., and "d" profiles. The F1 and F2 types have pleomorphic vesicles and form symmetrical synapses with the neuronal perikarya, the proximal dendrites and their spines, as well as with the initial axonal segments. The SR, LR1 and LR2 types contain round or oval vesicles and form asymmetrical synapses mainly with middle sized and small dendrites, and their spines. The d.c.v. boutons contain a mixed population of clear synaptic vesicles and dense core vesicles. The d.c.v. type forms asymmetrical synapses. The "d" profiles share identical features with the vesicle containing dendrites. The F2, SR, LR1, LR2, and "d" profiles take part in synaptic diads and/or triads, and occasionally participate in the synaptic glomeruli. The LR1 and LR2 take part in glomeruluslike formations. The Sth has a distinct synaptic pattern that permits its discrimination as a separate ultrastructural entity. The Sth seems to share some common ultrastructural features with the thalamic nuclei. On the other hand, the ultrastructural aspects of the Sth are much more different from its closest embryological allies: the both pallidal segments, and the zona reticulata of the substantia nigra.