Figure 6. The effect of intrinsic efficacy on tolerance after 7-day intermittent opioid agonist administration. Mice received subcutaneous injections every 24 h for 7 days with morphine, etorphine or fentanyl. The daily injection dose represents a multiple (top left of each panel) of the median effective dose (ED50) for morphine, etorphine, and fentanyl, as determined previously (Table 1). Controls received injections of saline. Twenty-four hours after the last injection all mice were tested in cumulative dose-response studies (tail flick) using the same drug. For ease of comparison, dose is presented on a scale that is equated for distance between units for each drug. Each panel represents a separate experiment for each drug (Table 5). The cumulative dose range for each drug for the dose response was morphine 1–18 mg *symbol* kg sup -1, etorphine 0.5–11.25 micro gram *symbol* kg sup -1, and fentanyl 10–240 micro gram *symbol* kg sup -1.