From experience gained in the monitoring of ovarian responses to gonadotropin therapy, it has been possible to develop concepts concerning the mechanisms which operate during the normal menstrual cycle. It is shown that the ovarian requirement for FSH operates in a very narrow range, involving changes in concentration of only 10%; this range operates from a threshold level, through an intermediate level to a maximum level. After adequate stimulation, the follicle goes through a costing phase during which it is independent of further FSH stimulation and it acquires sensitivity to LH. Methods for determining optimum requirements for FSH and HCG for the induction of singleton pregnancies are presented. The relevance of the findings in providing hypotheses explaining some aspects of ovulatory and anovulatory cycles are discussed.