The present invention provides compositions of matter which allow for the extended release and enhanced bioavailability of biologically-active polypeptides following parenteral delivery to an animal. More particularly, it concerns compositions comprising biologically-active somatotropin formulated for extended release, methods of preparing these compositions, and methods of using the same. These compositions comprise somatotropin, a pH-adjusting constituent (PAC), and a substantially non-aqueous, hydrophobic excipient. The PAC may comprise any suitable, biocompatible compound including, but not limited to one or mixtures of two or more of the following: acetic acid, phosphoric acid, monobasic phosphate. The PAC is added to the compositions of matter in an amount effective to adjust the pH to or near to the isoelectric point (pI) of the somatotropin component.