Rats with unilateral intrauterine silastic devices (made from 60 to 100 mg indomethacin/3 g silastic mixture) were found to have regular estrous cycles. No interference with body weight gain occurred in animals with the 100 mg/mix devices. The patterns of indomethacin release were dependent on the initial loading of the drug in the device, with the 60 and 100 mg/mix devices having delivery periods of approximately 50 days and 120 days respectively. Nevertheless, the absolute quantities of indomethacin delivered at any chosen time was remarkably similar for the two different loadings, suggesting that the properties of the external environment govern the total amount of drug released at specific times from these systems in utero.