Following the rapid progress in molecular biology, we are moving from the period of "one peptide a year" to a period of "one peptide a day". To cope with the rapid emergence of novel biologically active peptides, new techniques are needed to clarify their physiological functions. Evidence is presented here to show that antibody microinjection is a rapid and reliable method to evaluate the functions of endogenously released peptides. It is especially useful when no specific receptor antagonist is yet available for pharmacological study.