Silastic implants filled with progesterone in arachis oil were designed to provide a convenient and reliable method for the delivery of physiological concentrations of progesterone to mice. Placement of the implants in ovariectomized mice resulted in a rapid increase in circulating progesterone within 6 h; stable levels could be maintained for many days. Removal of the implants resulted in a very rapid fall in progesterone concentrations. The delivery of progesterone from the implants could be controlled by varying both the length of the implants and the concentration of progesterone internally. This allowed plasma progesterone concentrations to be maintained and controlled over the entire physiological range.