The antennal lobe of the brain of Manduca sexta comprises a central area of coarse neuropil surrounded by dense, spheroidal glomeruli, where all synaptic interactions between antennal-nerve axons and the second-order neurons of the lobe occur. Neuronal interactions in the glomeruli are complex, involving several types of neuritic profiles and mediated by synapses with a one-to-many ratio of pre- to postsynaptic elements. Presynaptic profiles in the glomeruli have been categorized into three types, containing round clear vesicles, large numbers of large dense-cored vesicles, and pleiomorphic clear vesicles, respectively. Preliminary studies of horseradish peroxidase-filled axons and neurons indicate that antennal-nerve axons form synapses without large numbers of dense-cored vesicles and that antennal-lobe neurons not only receive synapses but also may synapse onto other elements in the antennal lobe. A typical synaptic contact involves multiple postsynaptic elements apposed in pairs to an individual presynaptic element. The presynaptic element contains a bar-shaped membrane-associated density, which follows a shallow groove in the membrane and is flanked by synaptic vesicles. Postsynaptic elements are lined by membrane-associated densities in the region opposite to the synaptic bar, and may be observed to participate in serial synapses. Freeze-fracture replicas of the glomerular neuropil contain many membrane specializations that are thought to be presynaptic, some of which resemble those of vertebrate excitatory synapses. At these apparently presynaptic regions, large particles cluster in the P face of the membrane and are often surrounded by plasmalemmal deformations presumably representing sites of exo- or endocytosis. The shape of the predominant type of presynaptic membrane specialization (a plaque) does not match the shape of the presynaptic membrane-associated density (a bar); this raises the possibility that vesicle release occurs at isolated `active zones' along the presynaptic bar. Postsynaptic sites are represented by clusters of large particles in the E face of the postsynaptic membrane.