Methods and compositions useful for enhancing the absorption and/or transport of peptides, peptidomimetics, and other gastrointestina transport protein substrates through gastrointestinal transport proteins are provided. The methods comprise using hormones such as melatonin to increase the transport of the peptides, peptidomimetics, and substrates. The compositions comprise one or more hormones and one or more peptides, peptidomimetics, and other gastrointestinal transport protein substrates.