Nerve endings (n = 1,298) on noradrenaline cells in adrenal medullas of male rats (n =117) were investigated by quantitative electron microscopy, with sampling at 8 times during a standardized light:dark (12:12) photo-period cycle. Numbers per unit area were determined for synaptic vesicles of two types, small clear vesicles (SCVN) and large granular vesicles (LGVN); from these the percent of LGV per total vesicle number was calculated. In non-operated animals mean SCVN had two cycles per day, with acrophases near the middle of the light and dark phases and troughs near the times of change in lighting; there were also significant peaks (at start and middle of dark phase) and troughs in mean LGVN. Sham-operated (SPX) animals had decreased or insignificant signs of rhythmic changes in these two vesicle types; reduction was most marked in SCVN after SPX. Pinealectomized (PX) differed from non-operated animals in apparent losses of the cyclic changes in SCVN in the dark phase and in LGVN at the start of the dark phase; the former could have been due to a desynchronization of the dark phase rhythm in SCVN in the absence of the pineal. Nerve endings on A-cells and N-cells differed consistently and significantly in the quantitative relations and rhythmic patterns of their two major kinds of synaptic vesicles. These differences, and those seen after pinealectomy or sham surgery, indicate that both the neuroendocr¡ne and preganglionic mechanisms regulating the chromaffin cells have temporal and quantitative characteristics that differ in relation to major type of chromaffin cell affected