Four experiments, all employing the conditioned suppression of licking in thirsty rats, examined the extent to which reinforcement of one component, A, of a compound conditioned stimulus, AB, would "block" conditioning to the other element, B, on the first compound trial. Suppression to B was unaffected by prior reinforcement of A; that is, no evidence of blocking was obtained. If additional reinforced trials were given to the AB compound, further conditioning to B was blocked by prior reinforcement of A. Thus blocking appeared to take at least one trial to develop. These results suggest that blocking is not due to any competition between stimuli for association with reinforcement, but is a consequence of the animals' ignoring the added element once they have learned that it is redundant.