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The key pecking of two pigeons was reinforced with food on a progressive‐ratio schedule, which required an increasing number of responses for each successive reinforcement: 8, 16, 24, 32, etc. When the subject failed to complete the next ratio in the sequence within 60 min, the session terminated. The number of responses in the final completed ratio was defined as the “breaking point”. After the breaking point had stabilized (60 sessions), it served as a baseline to assess the effects of varying doses (5 to 80 mg/kg) of chlordiazepoxide and phenobarbital, administered intramuscularly 30 min before the sessions. Both drugs increased the breaking point. The dose‐effect curves were inverted U‐shaped, with maximum enhancement of performance occurring at 20 mg/kg for chlordiazepoxide and at 40 mg/kg for phenobarbital. A comparable enhancement was not obtained during a non‐drug “probe” session, which was conducted after the subjects' body weights had been temporarily reduced from 80% to 70% of their free‐feeding weights. The drug‐induced enhancement of breaking point was related to the initial values of the performance and may represent a reduction in the aversiveness of the schedule. 1972 Society for the Experimental Analysis of Behavior

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Journal of the Experimental Analysis of Behavior

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