Diurnal variations in the number of various types of small granulated and non-granulated vesicles as well as effects of continuous light on the vesicles in sympathetic nerve endings of the mouse pineal were investigated using quantitative electron microscopic techniques. Small granulated vesicles, 40 to 80 nm in diameter, were classified into two categories, type 1 or type 2, according to the size of the granule. Small non-granulated vesicles were also divided into two categories, i.e., spherical and elliptical. Non-granulated vesicles with the long diameter more than twice the shorter one (ranging from 50 to 90 nm in long diameter) were referred to as elliptical vesicles and the other (ranging from 40 to 80 nm in diameter) as spherical vesicles. The number of small vesicles, except for the non-granulated spherical vesicles, varied diurnally in relation to the daily photoperiod. The total vesicle density also exhibited diurnal changes. On the basis of these observations, possible relationships between small vesicles and noradrenaline or serotonin were discussed. Both the diurnal variation in small vesicles and that in the total vesicle density were completely abolished by continuous lighting. Continuous light exposure for various periods until 35 days provoked no apparent changes in small vesicles, except the type 2 granulated vesicles, which were increased in number in response to continuous lighting for about 1 to 3 days.