Three experiments examined developmental changes in the reversal of a learned discrimination in an olfactory conditioning paradigm. Preweanling rats were exposed to an odor (CS+) paired with footshock, and a different odor (CS-) that was explicitly unpaired with footshock. They were then immediately tested for their preference between the CS+ and CS- odors. In Experiment 1, animals trained at 12- or 18-days of age showed equivalent conditioned aversions to the CS+. When the significance of the cues was reversed the younger animals showed rapid reversal learning, i.e. their aversion to the original CS+ was displaced by an aversion to the original CS-. But the 18-day-old animals were slower to acquire reversal of the discrimination. Experiment 2 showed a similar pattern of results when animals received separate preference tests involving the CS+ vs a novel odor, or the CS- vs a novel odor. In Experiment 3, intact 18-day-old rats and 18-day-olds with damage to the septum and fornix showed equivalent conditioned aversions to the CS+.