An effective liquid preparation achieves high bioavailability (BA) of physiologically active peptides or proteins, including ghrelins, that are administered as drugs. Also provided is a method for improving the BA of physiologically active peptides or proteins, including ghrelins, that are subcutaneously injected in aqueous solutions. The liquid preparation contains: a physiologically active peptide or protein, such as ghrelins, as an active ingredient; an acid solution including one or a combination of two or more selected form the group consisting of acetic acid, lactic acid, phosphoric acid, glycine, citric acid, hydrochloric acid, propionic acid, butyric acid, benzoic acid and salts thereof; an alcohol; and a polar organic liquid including one or a combination of two or more selected from the group consisting of N-methyl-2-pyrrolidone, dimethylformamide, dimethylsulfoxide and methylparaben.