A sink in vitro drug elution system was applied to characterize the mechanisms and rates of drug release of progestins from silicone capsules. A zero order (constant) drug release profile was observed. The rate of drug release was found to be a linear function of the partition coefficient of drug from silicone polymer to elution medium. The subcutaneous implantation of such delivery device in heifers resulted an in vivo release rate in agreement with the value predetermined on the basis of in vitro data. The in vivo drug release rates observed for all the four progestins investigated were found to be computable from the physico-chemical parameters following a theoretical expression. The thermodynamics of the drug release process was also analyzed and found to be in agreement with experimental observations.