In this work we present information about the identification of opioid
peptides and receptors in the nervous system and other tissues of amp
hibians. Evidences about the possible participation of opioid systems
in the amphibians physiology are included. The importance of the studi
es of opioids in amphibians is considered in relation to significance
of these animals in the evolution of vertebrates, and their utility as
a study model for opioids due to their simplicity and plasticity, par
ticularly in the development and nociception process.