The present invention provides compositions 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 bioavailability-enhancing constituent (BEC), and a substantially non-aqueous, hydrophobic excipient. The BEC may comprise (i) amino acids or amino acid derivatives, such as histidine-HCl; (ii) hydroxamate derivatives, such as histidine hydroxamate or suberohydroxamic acid; (iii) non-reducing carbohydrates, such as trehalose or trehalose octaacetate; (iv) oxo-acid salts, such as a mixture of monobasic and dibasic sodium phosphate; or (v) a mixture of two or more compounds from within the foregoing classes (i)-(iv).