Biodegradable microcapsules may be utilized by injection to deliver drugs over a period of several weeks or more. These systems are especially applicable to highly potent drugs, such as certain peptides and steroids. Peptide-containing microcapsules are frequently fabricated by an organic phase separation technique. This consists of in situ precipitation of the polymeric component and the peptide from a solution of the polymer in a suitable solvent. The resultant microspheres may be injected intramuscular or subcutaneously following processing and suspension in a suitable aqueous vehicle. The polymers most frequently employed are the polyesters. Various molar ratios of lactide:glycolide in the form of poly ( d,l- lactide-coglycolide) have been found useful as matrices for peptides and steroids. Poly ( d,l- lactide-co-glycolide) has elicited negligible toxicity or inflammatory response in studies carried out to date. The irritancy of these materials is comparable to an IM dose of saline. These delivery systems are relatively complex. Interactions between drugs and polymeric components may occur. Amines have been noted to accelerate the hydrolysis of polyesters. It has been found that the degradation of some peptides is exacerbated in the presence of polyesters. Drug release of water-soluble compounds, such as peptides occurs in a biphasic manner. There is an initial drug burst in the first day or two, followed by a minimum. A second release phase begins as the polymeric matrix starts to degrade. Plasma levels attainable are in the ng/ml range levels in animals and, in certain cases, efficacy has been maintained for one to two months.