A sustained-release preparation which comprises a physiologically active peptide of general formula ##STR1##

wherein X represents an acyl group;

R₁, R₂ and R₄ each represents an aromatic cyclic group;

R₃ represents a D-amino acid residue or a group of the formula

##STR2##

wherein R₃ is a heterocyclic group;

R₅ represents a group of the formula —(CH₂)_n—R₅ wherein n is 2 or 3, and R₅ is an amino group which may optionally be substituted, an aromatic cyclic group or an O-glycosyl group;

R₆ represents a group of the formula —(CH₂)_n—R₆ wherein n is 2 or 3, and R₆ is an amino group which may optionally be substituted;

R₇ 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.

The sustained-release preparation shows a constant release of the peptide over a long time and is substantially free from an initial burst.