The axo-somatic terminals which synapse with the large spherical cells in the antero-ventral cochlear nucleus of the guinea pig are subdivided into three groups by means of different stereological form of their synaptic vesicles. Unilateral stimulation as well as long-term degeneration of the cochlear nerve produce significant changes of the cross sectional area of the terminals and the number of their mitochondria, when compared to the contralateral control. Both parameters are increased after stimulation and decreased following degeneration. These changes occur in different extent in the three groups of terminals. The discussion of these findings has led to the following conclusions: (1) The volume of synaptic terminals and the number of the enclosed mitochondria display the most pronounced plastic changes. Therefore these parameters seem to be very indicative for the study of function-related changes. (2) The volume of terminals, the quantity of their mitochondria, and the total sum of membranes may be increased under stimulation by actual acceleration of the axoplasmatic flow rate and by incorporation of the preterminal portion of the axon. (3) The presence of at least three types of terminals is evident from the different form of their synaptic vesicles and in addition, from their different response to our experimental conditions. It is not decided from the present findings, whether the bouton-like terminals and bulbs of the afferent cochlear nerve axons contain the same type of synaptic vesicles or whether these axons produce heterotypic terminals.