The present invention provides compositions comprising biologically-active somatotropin formulated for extended release and methods of preparing and methods of using the same. These compositions comprise somatotropin, at least a first bioavailability-enhancing constituent (BEC), and a substantially non-aqueous, hydrophobic excipient, and optionally a second BEC. The first BEC is typically a surfactant (preferably a non-ionic surfactant) or a cyclodextrin compound. The optional second BEC may comprise (i) amino acids or amino acid derivatives; (ii) hydroxamate derivatives; (iii) non-reducing carbohydrates; (iv) oxo-acid salts; or (v) a mixture of two or more compounds from within the foregoing classes (i)-(iv).