Interneurons of the cochlear granule cell domain are the target for nonprimary auditory inputs. The inputs to granule cell domains are in the form of large, mossy fiber terminals that form the core of synaptic glomeruli. The fact that mossy terminals are ultrastructurally heterogeneous indicates that the terminals may be functionally heterogeneous even when they innervate the granule cell domain through similar glomeruli. In the present study, we examined the synaptic relationships between a heterogeneous class of large synaptic terminals called mossy fibers and their targets within subdivisions of the granule cell domain. By using electron microscopic and immunohistochemical methods, we provide evidence for different dendrite classes that receive input from the mossy fibers. The distinct synaptic relations between mossy fibers and members of each neuron class further imply fundamentally separate roles for processing acoustic signals. Glutamate receptors mediate most excitatory synaptic transmission to the granule cell domain, but their activation in developing neurons also influences developmental processes. Here we have studied agedependent changes in the expression of AMPA receptor subunits in the cochlear nucleus. GluR2 and GluR3 subunits were expressed in the first 4 postnatal weeks. Age-related changes in GluR2 and GluR3 labelling in the CN between P10 and P40 were characterized by a progressive increase in the staining intensity without major modifications of the staining pattern.