It has been shown that there are several subtypes of κ opioid receptor. We examined ligand binding profiles and the effects of various opioid agonists on high potassium-stimulated release of [³H] histamine. We have evaluated the properties of non-μ, non-δ, binding of [³H] DIP ([³H] diprenorphine), anonselective opioid antagonist, in rat cortex membranes. Binding toμ and δ sites was inhibited by the use of an excess of competing selective agonists (DAMGO, DPDPE) for these sites. (-) Ethylketocyclazocine (EKC), DIP and bremazocine inhibited [³H] DIP binding. However, arylacetamides (U69593 and U50488H) gave little inhibition Replacement of sodium by NMDG and the addition of guanine nucleotide influenced the inhibitory potency of (-) EKC, an agonist for κ₁-and-κ₂-binding site, but not of bremazocine. This result suggests that bremazocine can be an antagonist at this binding site. Also, we have examined the opioid modulation of K⁺(30mM)-induced [³H] histamine release in rat frontal cortex slices labeled with 1-[³H] histidine. The [³H] histamine release from cortex slices was inhibited by EKC in a concentration-dependent manner. However, the δ receptor selective agonists, DPDPE and deltorphine II, μ receptor agonists, DAMGO and TAPS, κ₁-agonists, U69593 and U50488H, and ε-agonist, β-endorphin, did not. The concentration-response curve of EKC was shifted to right in the presence of naloxone, nor-binaltorphimine and bremazocine, respectively. These results suggest that κ₂ opioid receptor regulates histamine release in the fromtal cortex of the rat.