Studies on human ovulation are described, in which follicular growth and ovulation were controlled by HMG, clomiphene and HCG. Three regimes of treatment were employed: several injections of HMG, followed by an injection of HCG, a course of clomiphene followed by an injection of HCG and an injection of HCG alone timed to just pre-date the surge of LH during the natural cycle. Aspects of follicular growth and the timing of ovulation have been studied following the use of these treatments. Follicles rupture some 37 hr after the injection of HCG in cycles where HMG or clomiphene were used to stimulate follicular growth. The timing of ovulation was established with less precision when HCG was used to induce ovulation in the natural cycle, since the LH surge sometimes occurred before HCG was injected and other problems arose during luteinization. The numbers of follicles growing in response to stimulation with clomiphene or HMG showed only a slight relationship to the levels of oestrogen excreted in urine. The morphology of the preovulatory follicle was noted and some biochemical and physiological parameters were measured, including the levels of various steroids and prostaglandins in follicular fluid. The morphology of the oocyte and granulosa cells was recorded. These observations are discussed in relation to several physiological and clinical aspects of ovarian activity: the regulation of follicular growth, the timing of ovulation following the LH surge, factors affecting ovulation, and the relation of studies on follicular growth and ovulation to the rhythm method of contraception and other medical situations.