The movements of the presynaptic endocytotic structures produced during tetanic stimulation at 10 Hz were examined morphometrically and cytochemically in the cat superior cervical ganglion in vivo. The longitudinal profiles of the axon terminal and preterminal area were subdivided into five zones, I-V. Zone I, the area adjacent to the active zone, was assigned a hemicircle with a diameter equivalent to the active zone width (2R). Zones II-IV were defined by subdividing successively the presynaptic and preterminal areas within hemicircles with diameters equivalent to three-, five-, and sevenfold of 2R, respectively. Zone V was composed of the rest of the preterminal profile. The endocytotic structures, macropinocytotic endosomes and coated vesicles, observed in each zone were morphometrically analyzed with the time course of stimulation. The lateral surface of zone II was shown to be the main site for internalization of the terminal surface membrane during transmitter release. A large amount of the plasmalemma of zone II was rapidly retrieved by macropinocytosis to produce early endosomes at an increased rate of about three times that at rest. The population of coated vesicles, few in number at rest, increased to two- to threefold in zones II-V following the stimulation. Cytochemical examinations showed the incorporation of HRP into synaptic vesicles, endosomes and multivesicular bodies. An antibody against synaptophysin labeled the presynaptic endosomes, multivesicular bodies, coated vesicles as well as synaptic vesicles. The results have suggested that a considerable part of these endosomes was transported retrogradely at an increasing rate from zone II to zone V via zones III and IV. On the other hand, synaptophysin was observed to be distributed on the tubular protrusions of the presynaptic endosomes, suggesting the segregation and recycling of a part of synaptic vesicle proteins from the early endosomes in the nerve endings of the cat superior cervical ganglion.