To explore the role of stress in the HEB/LEB differences, we performed two additional experiments (see Supporting Text). HEB and LEB mice were tested in the hole-board, and corticosterone levels were determined immediately after the test or from untested animals in the room. Untested HEB and LEB mice did not differ in corticosterone levels (HEB, 4.4 ± 1.4 μg/dl; LEB, 4.4 ± 1.4 μg/dl; not significant). Both genotypes showed significant elevations in corticosterone after exposure to the task, but the increases were similar (HEB, 16.6 ± 1.7 μg/dl; LEB, 15.5 ± 2.1 μg/dl; not significant). Thus, although stress clearly accompanies the hole-board exploration test, it cannot explain the genotypic differences in this behavior. We also tested separate groups of HEB and LEB mice in the novel environment exploration task after habituating them for 23–24 h in the familiar compartment, rather than the 15 min we used in the other experiments. In this task there was again no significant genotypic difference in corticosterone levels that resulted from free exploration of the novel compartment (HEB, 8.6 ± 1.6 μ/dl; LEB, 6.4 ±
