The concept of multiple opioid receptors reconciles a large body of clinical and pharmacological data. Recent studies have shown that there are also multiple opioid binding sites. It would appear that there is considerable variability between species in both the specificity and selectivity of opioid receptors. This issue has not been explored systematically regarding opioid binding sites. Better characterization of the pharmacological profiles and receptor binding specificity for different species may help resolve some of the apparent disparities. The number of putative receptors now number nearly a dozen. Already subspecies of mu, kappa, and sigma receptors are being postulated. Both pharmacological and neurochemical methods may reveal even more. Some of the newer kappa agonists differ in their pharmacology from the prototypic kappa agonist ethylketazocine.