Three types of synapses occur in the anteroventral thalamic nucleus (AVN). Type 1 consists of small (0.5-0.8 microns) axonal endings densely packed with spherical synaptic vesicles. They form markedly asymmetrical synaptic contacts with distal portions of dendrites. Degenerative changes in these axons following destruction of the fornix identify them as the endings of the subicular projection to AVN. Type 2 synapses consist of large (1.0-1.5 microns) axonal processes containing spherical vesicles which form asymmetrical synapses on more proximal dendrites. Type 3 endings consist of large unidentified processes containing spherical, and occasionally flattened, synaptic vesicles forming symmetrical contacts with the largest stem dendrites. Neither of these synaptic types were modified by fornix lesions. The synaptic arrangements within AVN are simpler than other thalamic nuclei in that serial synapses and synaptic glomeruli are not present.