In order to understand the special requirements for the assessment of human toxicity for hormone releasing intrauterine devices it is necessary first to describe the constitution of one such system. The uterine progesterone system developed in our laboratories is the hormonal intrauterine system which has been most extensively tested clinically. It releases 50 μg per day of progesterone in continuous form. It has been designed to have a one-year period of functional life and it constitutes the first once a year hormonal fertility control agent and the first hormonal system which is target specific. A number of unique features in the design and construction of this new and intrinsically safe hormonal system are described.