Both the levels and types of binding sites for opiates and enkephalins appear to be different between species. The most dramatic differences are seen between rats, which have significant levels of both high and low affinity sites, and goldfish, which have only low affinity sites. Binding to both high and low affinity sites is easily displaced by low concentrations of morphine and D-Ala2-D-Leu5-enkephalin, suggesting that both represent relevant receptor sites.