Employed 24 paid graduate and undergraduate students to make a within-S comparison of 2 related procedures for negatively correlating a signal, or conditioned stimulus (CS), with a shock unconditioned stimulus (UCS) in differential conditioning of electrodermal responses (EDRs). One operation presented a nonreinforced stimulus (CS-) during reinforced trials for a 2nd stimuls (CS+). The other reinforced a stimulus (CS+) whenever it occurred alone but never reinforced a compound composed of that stimulus and a 2nd (inhibitory) stimulus (CSD). The conclusion that the CS- and CSD developed inhibitory properties was based on 3 classes of results: (a) during acquisition, there was clear differential responding to stimuli positively and negatively associated with the UCS; (b) on transfer trials, smaller responses were given to compounds of excitatory and inhibitory stimuli than to the excitatory stimuli alone; and (c) subjective judgments of expectation of shock paralleled the EDRs.