Abstract The mechanism responsible for development of tolerance to opiate-induced analgesia was investigated by means of an analysis of a series of dose-response curves in groups of rats which had developed different degrees of tolerance. Increasing tolerance was reflected in the curves by an increasingly large shift to the right and depression of the maximum effect. When the responses produced by morphine were plotted against the concentrations of the drug in the brain, instead of against the doses applied, and the curves analysed with a double reciprocal (Lineweaver-Burk) plot, it became evident that the K D value of morphine was not changed during tolerance development. The K D value obtained with this behavioral method was close to that obtained with receptor binding studies performed under identical conditions in vivo. The data implies that the mechanism(s) responsible for tolerance development to opiate-induced analgesia does not involve a qualitative change in the opiate binding sites. Instead, tolerance seems to be due to non-competitive changes in the transduction process between receptor occupation and response.