The present invention relates to an apparatus and method for treating erectile dysfunction. One of the causes of erectile dysfunction is venous leakage, where one or more veins carrying the blood from the penis are unable to retain sufficient blood within the penis to maintain erection. Embodiments of the present invention propose implanting contractile tissue sphincters around the base of the penis and/or around one or more veins, and stimulating the contractile tissue to contract when an erection is required to be maintained. Stimulation is provided by an implantable stimulator.