Although it is currently accepted that the extinction effect reflects new context-dependent learning, this is not so clear during infancy, because some studies did not find recovery of the extinguished conditioned response (CR) in rodents during this ontogenetic stage. However, recent studies have shown the return of an extinguished CR in infant rats. The present study analyzes the possibility of recovering an extinguished CR with a reinstatement procedure in a fear conditioning paradigm, on PD17 (Experiments 1 -4) and on PD24 (Experiment 5), while exploring the role of the olfactory content of the context upon the reinstatement effect during the preweanling period. Preweanling rats expressed a previously extinguished CR after a single experience with an unsignaled US. Furthermore, this result was only found when subjects were trained and tested in contexts that included an explicit odor, but not in standard experimental cages. Finally, Experiment 5 demonstrated the reinstatement effect on PD24 in a standard context. These results support the notion that extinction during infancy has the same characteristics as those described for extinction that occurs in adulthood. Instead of postulating a different mechanism for extinction during infancy, we propose that it may be more accurate to view the problem in terms of the variables that may differentially modulate the extinction effect according to the stages of ontogeny.Extinction can be defined as a procedure consisting of repeated presentations of a conditioned stimulus (CS) after conditioning (Pavlov 1927). As a result of this training, the conditioned response (CR) usually decays progressively. It is currently accepted that the extinction effect reflects new learning, because the extinguished CR can be recovered after a retention interval (spontaneous recovery), after the presentation of the unconditioned stimulus-the US, (reinstatement), or after a context changeknown as the renewal effect (Pavlov 1927;Bouton 2002;Quirk and Mueller 2008). All of these findings have been interpreted as evidence that the extinction effect is a context-dependent phenomenon (Bouton 2002(Bouton , 2004.Although extinction has been widely studied in adult organisms, only a few studies have focused on this effect during infancy, and these have yielded inconsistent results. In particular, in some of these studies the authors did not find recovery of the CR once it was extinguished (Kim and Richardson 2010), while others have reported renewal, more rapid reacquisition, and reinstatement in taste aversion learning (Revillo et al. 2014a), and spontaneous recovery (Revillo et al. 2014b) and renewal (Revillo et al. 2013 in fear conditioning. Those authors that did not find recovery of the extinguished CR suggested that in infancy, extinction is a qualitatively different phenomenon to that observed in adulthood, and that during infancy it results in erasure of the CS -US association instead of the production of new learning (Kim and Richardson 2010). However, more recent evidence of recovery a...