Disclosed is a method of manipulating the rate of gastrointestinal transit in a mammalian subject. Also disclosed is the use, in the manufacture of a medicament for the treatment of constipation, of a selective inhibitor of methanogensis, a methanogen-displacing probiotic agent, or a prebiotic agent that inhibits the growth of methanogenic bacteria or promotes the growth of competing non-methanogenic intestinal flora. Alternatively, in accordance with the invention, is disclosed the use in the manufacture of a medicament for the treatment of diarrhea, of methane or a methane precursor, a methanogenic or other methane-enhancing probiotic agent, or a methanogenesis-enhancing prebiotic agent.