The thermodynamic parameters for [3H]-ethylketocyclazocine binding in frog (Rana esculenta) brain membranes have been examined. Computer-based nonlinear regression analysis of the untransformed equilibrium displacement data showed that this ligand bound to two sites with different affinities and capacities in this tissue. KA values derived from equilibrium displacement curves have been used for calculating the changes in the standard Gibbs energy, enthalpy, and entropy during the binding process. Van't Hoff plots are bipartite, with transitions occurring at 18 degrees C for both the high- and the low-affinity sites. For the high-affinity site, the reaction appears to be associated with a decrease in enthalpy below the transition temperature and a significant gain in entropy above this temperature. The reverse appears to be true for the low-affinity site. We conclude that this profile fairly approximates the mixed agonist-antagonist nature of this ligand and surmise that thermodynamic analysis could be a very useful tool for characterization of the nature of cloned opioid receptors in vitro.