39 male rats were trained to depress a horizontal bar to obtain a food pellet. One group of 20 rats received 10 reinforcements, the other 19 received 40. Half of each group were extinguished (by non-reward) 1 day after training and the other half 15 days afterward. With both the 10- and 40- reinforcement animals, those undergoing extinction 15 days after training made more responses than those extinguished 1 day afterward. The increased resistance to extinction, which is similar to that found in the conditioned response, is described in terms of loss of habituation. (PsycINFO Database Record (c) 2006 APA, all rights reserved)