The ovulatory cycle in adult females appears to be a clock-driven process in which the regular gonadotrophin surges that induce ovulation are timed by the cycle of light and dark. In the absence of photoperiodic cues, the ovulatory surge appears to be driven by an endogenous rhythm, perhaps generated by the suprachiasmatic nuclei of the hypothalamus. There is now evidence that puberty onset is associated with the development of the capacity to reset biological rhythms in response to photoperiodic cues and to link neuroendocrine secretion to clock rhythms. This capacity emerges around 26-28 days of age in immature animals. A hypothesis is presented in which the initiation of responsiveness to photoperiodic signals is related to the onset of regular waves of follicular development in the ovaries that culminate in first ovulation 10 to 14 days later.