Results from a two-epoch catFISH experiment. catFISH for the immediate early gene Arc was performed in the CA1 region of the hippocampus of rats that had explored a novel environment, “A.” The caged controls were sacrificed directly from their home cages and showed a low percentage of neurons positive for Arc staining. By contrast, the A immediate group, which was exposed to A for 5 min and then immediately sacrificed, showed a dramatic increase in the percentage of neurons that contained Arc intranuclear foci. However, when the rats were returned to their home cages for 25 min after exploring A (A delay group), Arc intranuclear foci–positive cells returned to control levels, and the percentage of Arc cytoplasmic-positive cells had risen to 44%. These three groups establish the time course of Arc induction in CA1 neurons. Rats in the A/A group explored environment A for 5 min, were returned to their home cages for 20 min, explored environment A for 5 min more, and were immediately sacrificed. Rats from the A/B group were handled similarly to the A/A group, except that rats explored a different environment, “B,” in the second exploration session. While a similar percentage of cells was activated by either A or B environments, the distinct staining profiles for the A/A and A/B rats indicate that a single neuronal population was activated by the two exposures of the A/A rats and that two neuronal populations were activated by the two exposures of the A/B rats. The A/A and A/B groups demonstrate that the induction of Arc gene expression in CA1 neurons is specifically linked to neural encoding processes. n = 3 rats per group. All rats were sacrificed by decapitation. * p < 0.002, relative to caged controls (ANOVA); # p < 0.05, relative to the other two cell populations for that group (paired t-test). For a full description of this study, see Guzowski et al. (1999). Reprinted from Guzowski et al. (1999) with permission from Nature.