Changes in electrophysiological properties measured in vitro were correlated with ultrastructural differences at synapses between sense cells and the primary afferent neurons in electrosensory organs of the thornback ray (the ampullae of Lorenzini). Variation in synaptic structure was classified into four synaptic morphotypes, which appear to represent stages in a cyclic pattern of ultrastructural modification associated with changes in synaptic efficacy. Synapses with deeper postsynaptic troughs, and active zone regions located at the "narrow point" of the presynaptic evagination, and other morphological differences, were associated with greater sensitivity and spontaneous activity. Furthermore, the morphology of synapses was different in organs that had shown increasing, decreasing or stable trends in sensitivity prior to fixation, suggesting that changes in synaptic physiology and morphology are interrelated, and providing evidence for the sequence of ultrastructural modifications represented by the four synaptic morphotypes. These results support the conclusion that synaptic morphology is plastic and that this plasticity has functional significance in terms of the threshold sensitivity and spontaneous activity monitored from the afferent nerves. Plasticity of synaptic morphology which is associated with changes in the efficacy of transmitter release at chemically mediated synapses could be important in relatively long-term phenomena.