AbstractMaturation of Xenopus laevis oocytes can be induced by mianserine, a tricyclic antidepressant. K+‐free medium facilitates this maturation process. Mianserine must be kept in contact with the oocytes during the whole process of maturation for maximal efficiency. It is inactive after injection into the oocytes. Mianserine induces the formation of maturation‐promoting factor (MPF) in the treated oocytes. Mianserine‐induced maturation is strongly inhibited by theophylline, even in K+‐free medium. Progesterone displays synergistic effects with mianserine for the induction of maturation. Likewise, oestradiol shows cooperative maturing effects with progesterone as well as with mianserine. It is suggested that mianserine exerts its primary effects on oocyte maturation by inhibiting a membrane adenylate