Electron microscopical studies of the thalamic AV-AM nuclei substantiated the presence of two main types of neurons, i.e. principal (or relay) cells and Golgi type II interneurons. Characteristic synaptic islands are found in abundance in the AV-AM. Four different types of synaptic terminals have been identified in these islands: RL-boutons = large axonal terminals with round synaptic vesicles; RS-boutons = small axonal terminals with round synaptic vesicles; F1-boutons = small axonal profiles containing flattened synaptic vesicles, and F2-profiles interpreted as presynaptic dendrite appendages, bearing pleomorphic vesicles, both belonging to the Golgi type II interneurons. --The synaptic relations were studied in normal preparations and after lesions in the mamillary body, limbic cortex and hippocampus. The specific afferents (RL-boutons)--originating from the medial mamillary nucleus--are presynaptic to both relay cell dendrites and "presynaptic" dendrite profiles of Golgi type II interneurons, which in turn are presynaptic to the same relay dendrites (synaptic triads). RS-boutons originate mainly from limbic cortex and hippocampus.