In the course of previous studies using continuous monitoring of the electrophysiological correlates of GnRH pulse generator activity, characterized by episodic increases in hypothalamic multiunit electrical activity (MUA volley), it was noticed that the nocturnal slowing of pulse generator frequency was an acute phenomenon observable in the first MUA volleys after the lights were turned off, as was the increase in frequency when the lights were turned on in the morning. This suggested that the reduction in pulse generator frequency at night may not be the consequence of an intrinsic diurnal rhythm, but an effect of light per se. Indeed, as reported herein, such an effect was observed when the lights were turned on or off at times other than the normal illumination period (normal light schedule, lights on from 0700-1900 h). That this was not simply a response to arousal was shown by awakening the animals with loud recorded noises in total darkness at the same unaccustomed times without a resulting change in frequency. This suggests that the effect of light is specific, perhaps mediated by the retino-hypothalamic tract. This direct action of light, however, is superimposed upon a diurnal rhythm, as shown by a reduction in pulse generator frequency during the subjective night when the monkeys were kept in constant light or constant darkness.