Abstract Synapses are dynamic structures reflecting environmental events. It was observed that densities associated with apposing synaptic membranes were altered in response to changing conditions of auditory stimulation ( Gulley et al. 1978 . J. Comp. Neurol. 180: 707–742). Synapses also exhibit plasticity during early development of the auditory system ( Larsen and Pappas, 1985. Proceedings: 43rd Annual Meeting of the Electron Microscopy Society of America, pp. 493–494). Larsen and Pappas reported that, in young kittens, the end bulb of Held (EBH) terminal was distinguished by excessive density at apposing synaptic membranes which assume gradually the pattern observed in adult cats. The maturation of synapses parallels emerging function and may depend upon auditory stimulation ( Larsen and Kirchhoff, 1987. Neurosci. Abstr. 13: 1260). Larsen and Kirchhoff found that EBH synapses on large spherical cells of white-deaf cats resembled the immature synapses found in young kittens (Fig. 1). They also found terminals that had an increased number and length of the membrane densities associated with synapses. Because the number of synaptic vesicles in these presynaptic terminals was equivalent to the number of vesicles found in normal-hearing cats, Larsen and Kirchhoff (1987) suggest that this is evidence of synaptic plasticity. We have completed a quantitative study of the synapses of EBH terminals found in adult cats that had been deaf for at least 3 years.