Effects of repeated injections of naltrexone on antagonism of rate decreases by morphine in the pigeon.

Responding of three pigeons was maintained under a multiple schedule of food presentation in which key-pecks produced access to grain under a fixed-interval schedule in the presence of one stimulus and a fixed-ratio schedule in the presence of another stimulus. Repeated daily injections of 1 mg/kg naltrexone had no systematic effect on overall response rate during either schedule component, and the naltrexone continued to antagonize rate decreases of periodic single injections of 10 mg/kg morphine for seven weeks. Tolerance did not occur to the antagonistic effects of naltrexone on rate decreases generally produced by morphine.