The hypothesis that amobarbital causes an increase in output of behavior by inhibition of the effect of an inhibitory stimulus has been tested in comparison to the hypothesis that the differential increases are related to different probabilities of responding without the drug. Experiments were made on 3 pigeons pecking a plastic key under a schedule of positive reinforcement. It was found that the increase in rate of responding following amobarbital did depend on how fast the bird would have been responding in the absence of amobarbital. This relationship held whether or not an inhibitory stimulus was present. The dependency on control rate satisfactorily accounted for the greater mean increases in the presence of the inhibitory stimulus. There was thus no indication that amobarbital specifically inhibited the effects of the inhibitory stimulus.