It was recently reported that 19-norprogesterone pellets implanted into adult mice inhibit fertility (Folch Pi, Orlol, Herrera Lasso, Maqueo, Dorfman and Kind, 1965). The amount of the active substance, released from the steroid-cholesterol preparation was shown to be about 10 [mu]g/day. Such low amounts were apparently not sufficient to inhibit hypothalamo-hypophysial function and fertility control was achieved by other mechanisms. The present studies were undertaken with the purpose of establishing whether similar control of fertility could be achieved in rats and hamsters by pellet implants.