Sustained-release preparation

A method of producing a sustained-release preparation which comprises dissolving a physiologically active peptide of the formula ##STR23## wherein X represent an acyl group;

• R.sub.1, R.sub.2 and R.sub.4 each represents an aromatic cyclic group which may be the same or different;
• R.sub.3 represents a D-amino acid residue or a group of the formula ##STR24## wherein R.sub.3 ' is a heterocyclic group;
• R.sub.5 represents a group of the formula --(CH.sub.2).sub.n R.sub.5 ', wherein n is 2 or 3, and R.sub.5 ' is an amino group which may optionally be substituted, an aromatic cyclic group or an O-glycosyl group;
• R.sub.6 represents a group of the formula --(CH.sub.2).sub.n R.sub.6 ' wherein n is 2 or 3, and R.sub.6 ' is an amino group which may optionally be substituted;
• R.sub.7 represents a D-amino acid residue or an azaglycyl residue;
• and
• Q represents hydrogen or a lower alkyl group,
• or a salt thereof, and
• a biodegradable polymer having a terminal carboxyl group wherein the number average molecular weight of said polymer by end-group determination is about 0.4 to 2 times of the number average molecular weight by gel permeation chromatography in a solvent which is substantially immiscible with water and then removing said solvent.