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To determine the effects of insufficient or excessive doses of human chorionic gonadotrophin (HCG) on ovulation, varying single doses from 5 to 100 IU were administered to 40 does in oestrus. A total of 371 preovulatory follicles and oocytes were observed. Low doses of HCG (5-10) started resumption of meiosis, but no ovulation occurred. Higher doses progressively induced nuclear maturation (prometaphase, metaphase I, metaphase II). Simultaneously increasing doses initiated ovulation of some of the follicles, then of most of the follicles. Unruptured follicles, mostly with oocytes in metaphase II were frequent and depending on the dose, these were preovulatory, haemorrhagic or luteinized. The administration of different doses demonstrated the possible dissociation of several mechanisms leading to ovulation. The induction of nuclear maturation requires lower doses of HCG than luteinization. Follicular rupture requires even higher doses. Premature luteinization induces intrafollicular ovum retention without (LUF syndrome) or with follicular rupture. These mechanisms of ovulation explain the effects of blunted luteinizing hormone surges or inadequate HCG administration.