Silicone hydrophobic elastomers, like polydimethyl siloxane (PDMS) are the most frecuently used polymers in commercial devices in controlled delivery systems for steroid molecules in order to control estrus cycle in livestock. Polyisoprene (PI) is another hydrophobic elastomer which was evaluated as Progesterone (P4) matrix for intravaginal administration. The goal of this work was to characterize the polymorphs of P4 deposited in different polymeric matrices in order to determine the correlation with in vivo and in vitro studies. Finally, it was important to characterize the effects of preparation conditions on the polymorphs present in order to improve the P4 release reducing the amount of drug loaded within the intravaginal devices.