Three experiments were conducted with rats to determine the effects of electric shock on responding during an operant discrimination. In two of these experiments, a conditioned suppression procedure was superimposed upon a stimulus signalling the availability of food reinforcement (SD). Response rates were greatly suppressed, not only in the warning signal periods which preceded each shock, but in the presence of SD, and the stimulus signalling the unavailability of reinforcement (SI) as well. A third experiment, in which a very mild shock was used without a warning signal, demonstrated an increased response rate in SD and SI, although this effect was rather unsystematic. In a similar study, Hearst (1965) found an increased rate in SA independent of any change in the SD rate. The present study failed to obtain Hearst's effect but illustrated a suppressive effect with a similar procedure.Pavlov (1927) reported that a respondent discrimination may be disrupted by making the physical differences between reinforced and unreinforced stimuli quite small. Others, notably Cook (1939) and Masserman (1943) have investigated the effects of "stress" on instrumental behavior. The results from these studies are similar to those found by Pavlov, including the well known emotional behaviors which accompany the disruption of the discrimination usually referred to as "experimental neurosis". All of the aforementioned studies involved a conditioned conflict procedure.Another type of procedure used to elicit emotional behavior has been developed by Estes and Skinner (1941 tion to which it is not directly related. In such an investigation, Hearst (1965) found that when the warning signal followed by shock was periodically superimposed upon a stimulus in the presence of which responding was reinforced with food (SD), unreinforced responding in the presence of another stimulus (SA), increased. This increased rate of behavior seems peculiar insofar as the conditioned suppression procedure nearly always decreases rates of responding. Hearst's finding raises the question: "When is the effect of the stimulusshock pairings not only suppressive but facilitative as well?"Facilitative effects of electric shock on consummatory behavior have been shown by several investigators (