Experiments were performed in order to obtain physiological evidence for the presence of delta opioid receptors in the guinea pig isolated ileum. The nonequilibrium narcotic antagonists beta-chlornaltrexamine (beta-CNA) and beta-funaltrexamine (beta-FNA) differentially affected inhibitory responses to mu- and delta-specific opiate ligands (q value). These results indicate that these agonists do not act through identical populations of receptor in the ileum despite the similarity of their naloxone Ke values (apparent naloxone dissociation constant determined physiologically). Moreover, elimination of myenteric mu receptors resulted in a 10-fold increase in the naloxone Ke for the highly selective delta agonist Tyr-D-Ser-Gly-Phe-Leu-Thr, suggesting the involvement of a second receptor subtype. It is suggested that this receptor is delta in nature.