Spinal cord motoneuron neuropil (cervical and lumbar enlargements) has been studied at the ultrastructural level after fixation with glutaraldehyde and staining with uranyl acetate and lead citrate. Based on the appearance of the synaptic specializations, different types of boutons were identified and correlated with the classification of bouton types based on osmicated tissue. The sulfide silver method for histochemical demonstration of heavy metals was applied to the same region. The localization of reaction products (silver grains) was predominantly in the terminals. Within the bouton, the grains were mainly in the specialized region of the synaptic contact, and the presynaptic network has also labelled, but to a lesser degree. All stained boutons had the same type of paramembranous synaptic specialization, but not all of the boutons with this type of specialization were stained. The stained boutons are interpreted as a fraction of the 'F' boutons.