The invention relates to a process of preparing pharmaceutical closures for containers made of flexible polymeric materials which are suitable to be filled with parenterally administerable fluids prior to their sealing and sterilization. The process involves a specific relaxation period for the elastomer material of the closure which thereby can settle and develop resealing characteristics. The closures will have an excellent resealing capacity after being penetrated by a piercing device for establishing fluid communication with the container, even after multiple entries into the container.