Effects of fixed-ratio requirement on observed tolerance to decreased responding by clonidine.

Responding of rats was maintained under either a 10- or a 40-response fixed-ratio schedule, and "local" rates of responding were 0.29-0.37 responses per sec for both schedules. Clonidine decreased responding for both schedules in a similar and dose-dependent manner, and the largest dose tested (0.3 mg/kg) completely suppressed behavior. When 0.1 mg/kg was administered immediately prior to 30 daily experimental sessions, FR10 responding recovered to control levels within 15 sessions, whereas FR40 responding recovered only to approximately 60% of control level at asymptote. These results continue to identify boundary conditions for the influence of reinforcer loss on tolerance development, and they emphasize the overriding influence of behavioral processes on observed tolerance to the behavioral effects of drugs.