Circadian rhythm of fluctuations in the level of gonadoliberin in the hypothalamus of rats and the influence on it of various xenobiotics.

There are no regular patterns in the fluctuations of the content of gonadoliberin in relation to the time of day in the preoptic region and the median eminence in females and males. However, the existence of a well-marked circadian rhythm of the fluctuations of this neurohormone has been identified in females in the vascular organ of the lamina terminalis. Its level is extremely low in the morning; its level increases five-fold immediately after noon, and ten-fold at 1700-1800 h.