Behavioral influences on tolerance to the effects of morphine on schedule-controlled behavior.

Responding of pigeons was maintained under a multiple fixed interval, fixed ratio schedule of food delivery, and 10 mg/kg morphine was administered daily. Responding during both schedule components was initially decreased and measurable tolerance developed to this effect after four daily injections. However, the rate of tolerance development differed depending on whether or not presence of the drug coincided with performance during experimental sessions. Tolerance developed more rapidly when morphine was given before daily experimental sessions than when morphine was given daily but animals did not perform daily in experimental sessions. Tolerance to the rate-decreasing effects of morphine depended on relations between presence of the drug and exposure to experimental sessions.