Reversible stereoselective complexes were spontaneously obtained from mixing acetonitrile solutions of enatiomeric D-poly(lactic acid) (D-PLA), L-poly(lactic acid) (L-PLA), and leuprolide, a L-configured nonapeptide LHRH analogue. The complex spontaneously aggregated and precipitated in high yields (>90%) from acetonitrile solution, forming uniform, porous microparticles. The stereocomplex microparticles showed a continuous release of the interlocked peptide for a period of one to three months under physiological conditions. Various factors, including method of complex formation, molecular weight of PLA, leuprolide: polymer and D-PLA:L-PLA complex ratios, and additives, influenced the release pattern of leuprolide from the stereocomplexes. Continuous release of leuprolide for over 100 days was observed for certain stereocomplex compositions. In vivo evaluation of the leuprolide loaded stereocomplexes in rats by monitoring testosterone levels in the blood of rats after subcutaneous injection showed low testosterone levels for over 42 days.