Two experiments using the conditioned suppression procedure were carried out in order to test the role of direct context-US association in differential inhibition. In Experiment 1, experimental groups differed in inter-trial interval (ITI) and shock density. The results showed that only the short ITI group (higher density shock) passed both retardation and summation tests. Experiment 2 was designed to test whether differences in several kind of ITIs and type of trial presentation could explain differences among groups undergoing the summation test. Results indicated that ITI is the critical variable implicated, but its contribution to inhibitory control of response is modulated by the type of trial presentation. Only the group with short ITIs and random presentation of trials passed both tests. The critical result was that there were no differences in contextual conditioning compared with its yoked group (trials in alternation). The results are discussed within the framework of the ambiguous expectancy hypothesis for differential inhibition proposed in this paper. Key words: Ambiguous Expectancy, Differential Inhibition, Expectancy of Reinforcement.