Changes in the pattern, number and size of synaptic vesicles during transmitter release were examined in the synapses of the cat superior cervical ganglion (SCG) in vivo in relation to alteration in the amplitude of postganglionic compound action potential (PGP). Stimulation of the preganglionic nerve fibers at 10 Hz caused an increase in the mean amplitude of PGP. It became augmented by approximately 30% compared to control 10-30 sec after starting the stimulation, and then gradually declined to reach a plateau after 4-6 min. This level, about 20% higher in value than control, was sustained until the end of 30 min of stimulation. The nerve terminal was divided into two areas to examine topographically the numerical changes in synaptic vesicles (SVs): zone I on the presynaptic membrane encircled with a diameter equivalent to the active zone length, and zone II occupying the remaining area outside zone I. The synaptic vesicle density in zone I (vesicle number/microns 2) was 96.9 +/- 4.8 (mean +/- S.E.M.) in the unstimulated control ganglia and 128.8 +/- 9.4 (mean +/- S.E.M.) in the ganglia stimulated for 10-30 sec, which was 30% higher in value than control. Then, it decreased slightly reaching a plateau, 20% higher in value than control. The diameter distribution of the SVs showed that their diameters in zone I (56.6 nm mean) were larger than those (51.6 nm mean) in zone II, and that prolonged stimulation induced smaller vesicles in both areas. The results showed that the increase in SV number in zone I correlated well with the elevation of PGP.(ABSTRACT TRUNCATED AT 250 WORDS)