It has been reported that peptides, condensed amino acids, may provide health benefits, such as anti-hypertensive, anti-diabetic, and anti-dementia effects. One successful TOKUHO product is a sardine peptide that can modulate high blood pressure. However, the bioavailability of bioactive peptides still remains unclear due to a lack of appropriate analytical assay systems. In order to detect absorbed analytes in a small volume of blood, highly sensitive analytical assays that allow detection at ＞pmol/mL-plasma, such as fluorescence-derivatization column-switching HPLC, amine-derivatization-LC-MS/MS, and MS imaging for visualization of tissue distribution have been developed. In this review, the bioavailability of small peptides as intact forms and their physiological potentials are discussed.