Trengestone is a steroid substance, a retroprogesterone. It has a strong progestogenic action and stimulating effect on the hypotalamo-hypophyseal system. It does not possess a direct oestrogenic or androgenic action. It indirectly stimulates the oestrogen productivity in the ovary via the central nervous system. The mechanism of action for induction of ovulation is not clear. It is indicated for the treatment of normooestrogenic anovulation. Ovulation occurs after trengestone administration either immediately ater termination of treatment or much later. Another indication for its administration are other functional disorders of the cycle and treatment of imminent abortion. Side-effects are: mastalgia, headache and fatigue.