The aim of this work was to develop injectable microcapsules containing leuprolide acetate, that would deliver that drug at a zero-order rate over a period of about one month. A large amount of leuprolide acetate, a hydrophilic drug, was entrapped into polylactic aicd (PLA) and copoly-(lactic/glycolic) acid (PLGA) microcapsules prepared by an in-water dring method using a (w/o)/w emulsion. The basic techniques were designed to increase the viwcosities of the inner water phase and w/o emulsion. Under the conditions used, it was possible for the drug to be completely entrapped by the microcapsules in the range of 10 to 20% on the basis of the polymer. However, the release profiles of the drug in vitro were inadequate for controlled release for one month.