Auditory nerve fibres make large excitatory synaptic contacts, the endbulbs of Held, with bushy cells in the anteroventral cochlear nucleus (AVCN). We have used serial-section electron microscopy to reconstruct seven endbulbs of Held in contact with three different AVCN bushy cells from a 25-day-old rat, as a basis for interpreting our previous physiological results at this connection. Four endbulbs of Held contacting the same bushy cell were completely reconstructed. The number of separate synaptic specializations within these endbulbs varied from 85 to 217, with a mean of 155. Detailed measurements were obtained from high magnification segments of four endbulbs contacting three different bushy cells. Large variability was found in the size of synaptic specializations within an individual endbulb. The size of postsynaptic densities (PSDs) varied between endbulbs (mean PSD area 0.03, 0.07, 0.07 and 0.18 microm(2); n = 4 endbulbs). The number of morphologically docked vesicles at individual specializations within the same endbulb varied considerably (between 1 and 102). The mean number of morphologically docked vesicles per specialization differed between endbulbs (mean numbers of docked vesicles per specialization = 2.1, 3.7, 5.3, 14.8; n = 4 endbulbs). Despite these large differences, the density of docked vesicles per square micron of PSD was similar between endbulbs (54, 80, 81, 83 docked vesicles per microm(2); n = 4 endbulbs). Within an endbulb, a linear relationship was found between the number of docked vesicles and PSD area, and between PSD area and the number of undocked vesicles within 150 nm of the active zone. The ratio of undocked vesicles (< 150 nm) to docked vesicles ranged from 2 to 5 in different endbulbs (n = 4 endbulbs). These structural observations are discussed in relation to the functional properties of synaptic transmission between endbulbs of Held and bushy cells in the AVCN.