Fetal and infant rats can learn to avoid odors paired with illness before development of brain areas supporting this learning in adults, suggesting an alternate learning circuit. Here we begin to document the transition from the infant to adult neural circuit underlying odor-malaise avoidance learning using LiCl (0.3 M; 1% of body weight, ip) and a 30-min peppermint-odor exposure. Conditioning groups included: Paired odor-LiCl, Paired odor-LiCl-Nursing, LiCl, and odor-saline. Results showed that Paired LiCl-odor conditioning induced a learned odor aversion in postnatal day (PN) 7, 12, and 23 pups. Odor-LiCl Paired Nursing induced a learned odor preference in PN7 and PN12 pups but blocked learning in PN23 pups. 14C 2-deoxyglucose (2-DG) autoradiography indicated enhanced olfactory bulb activity in PN7 and PN12 pups with odor preference and avoidance learning. The odor aversion in weanling aged (PN23) pups resulted in enhanced amygdala activity in Paired odor-LiCl pups, but not if they were nursing. Thus, the neural circuit supporting malaise-induced aversions changes over development, indicating that similar infant and adult-learned behaviors may have distinct neural circuits.Animals rapidly learn to avoid tastes, smells, and textures of foods associated with malaise, and this learning process produces conditioned taste and odor aversions (Garcia et al. 1966(Garcia et al. , 1974 Best 1992, 1993). This learning has been demonstrated early in development, including both the embryonic day (ED) 18 rat and mouse fetuses, with retention time lasting weeks Haroutunian and Campbell 1979;Smotherman 1982;Stickrod et al. 1982b;Rudy and Cheatle 1983; Robinson 1985, 1990;Alleva and Calamandrei 1986;Molina et al. 1986;Hoffmann et al. 1987Hoffmann et al. , 1990Miller et al. 1990;Hunt et al. 1991Hunt et al. , 1993Abate et al. 2001;Richardson and McNally 2003;Gruest et al. 2004a). Taste/odor aversion acquisition shows some specific differences between pups and adults. First, nursing during acquisition disturbs taste-aversion learning, although this diminishes as pups approach weanling (Martin and Alberts 1979;Gubernick and Alberts 1984;Melcer et al. 1985;Kehoe and Blass 1986). Second, in contrast to the long temporal parameters permitted between the odor/taste (conditioned stimulus, CS) and illness (unconditioned stimulus, US) in adult taste aversion, the temporal constraints between the CS and the malaise producing US for pup learning are very limited (Rudy and Cheatle 1983;Kraemer et al. 1988Kraemer et al. , 1989Hoffmann et al. 1990Hoffmann et al. , 1991.There is evidence that the neural basis of odor/taste aversion learning may change over development. In adults, the amygdala is thought to support taste-aversion learning, though there are discrepancies in the literature (Nachman and Ashe 1974;Burt and Smotherman 1980;Dunn and Everitt 1988;Yamamoto and Fujimoto 1991;Kesner et al. 1992;Yamamoto et al. 1994;Bures et al. 1998;Schafe et al. 1998;Sakai and Yamamoto 1999;Wilkins and Bernstein 2006). When the taste and smell component...

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